How Many Men Deserted in 408?

In his work on the fall of Rome, Peter Heather rejected a claim of Zosimus, an ancient historian. Zosimus wrote that 30,000 men abandoned the Western Roman Empire in 408, and joined the army of Alaric. The figure of 30,000, Heather opined, is "impossibly high." In Heather's view, the true number was around 10,000. However, available evidence suggests Zosimus was nearer the mark.
To arrive at a reasonable estimate of the number of deserters, we must determine the size of the army of Italy before the barbarian soldiers departed, and the number of soldiers remaining after they left.
Available figures, although scanty, suggest that prior to 408 Stilicho, the Roman commander in Italy, had approximately 30,000 soldiers. Few remained in the army after 408.
In 406 CE, when the barbarian horde of Radagaisus was attacking Italy, Stilicho mobilized about 15 numerii or regiments, totaling 15,000 men. He also called in soldiers from the Rhine, perhaps 5,000 men. In addition, there was a force of goths (commanded by an ally, Sarus) and some Huns. The latter two groups added perhaps another 5,000.
It is unlikely that Stilicho had fewer than 25,000 troops. He probably needed that many to beat the invaders. Even after losing a third of its troops, the army of Radagaisus still had 12,000 good fighters and perhaps 10,000 others who were sold into slavery (and later joined Alaric). To overcome such a force, by besieging it in the vicinity of Florence and Faesulae, Stilicho must have had at least as many soldiers, or 25,000 plus. And he retained most of his army after the battle.
Roman casualties were probably light because Stilicho relied mainly on trapping the enemy and starving him into submission. He might have lost a few thousand men at most, leaving him with over 20,000 of his original force. But after Radagaisus surrendered with his army, 12,000 of the best survivors were conscripted into the army of Stilicho. By late 406 it probably numbered over 32,000 troops. The events of 407 did not cost it many of these.
After the Vandals, Alans and Suevi invaded Gaul by 407, Constantine, a Roman general, crossed from Britain to the continent. He drove the barbarians back but lacked the strength to subdue or eject them. Constantine was proclaimed emperor (Constantine III) hence became an enemy of Honorius, the emperor Stilicho served under. To deal with the usurper, Stilicho sent Sarus to Gaul. Despite some initial success, Sarus was forced to flee back to Italy. We know little for sure, but it seems likely Sarus brought the goths he had commanded the previous year. This army was probably not very large and was not annihilated. Therefore, by 408 the army of Italy (or Stilicho) probably still had around 30,000 men.
Stilicho fell out of favor in 408, and was killed. The anti-barbarian faction responsible for his fall included native Roman soldiers. These men launched a pogrom against barbarian women and children, then quartered in Roman cities. The civilians were slaughtered wholesale. It is noteworthy that the native troops didn't attempt to kill the barbarian soldiers, just defenseless civilians. And they did so in cities, presumably safe within their walls. The native elements could not have been very numerous or strong, or they would've been more ambitious and courageous. This is good evidence--if more were needed-- that most of the army by then consisted of barbarian recruits.
Apparently there was little the barbarian soldiers could do to save their families or retaliate directly. But they were extremely angry. Since the Roman troops probably killed all barbarian civilians, regardless of whether they were Goths or something else, just about every barbarian soldier would've felt incensed and completely unwelcome. They could not trust the Romans and would no longer fight for them. Ergo, they left. We shouldn't assume they all joined Alaric; a majority of them probably just went home, to the laeti or barbarian settlements along the frontier. But virtually all would've abandoned their posts. But what evidence do we have that the deserters totaled nearly 30,000 (perhaps 26-27,000)? That would require the near total loss of the army of Italy.
In fact, that is exactly what seems to have happened. There is clear evidence that 408 CE was a watershed. Almost overnight, the Western Roman Empire went from military power to near  impotence:

• In 409, the year after the mass desertion, the government had to call in a force of 6,000 troops from Dalmatia to try to garrison Rome. Apparently the army of Italy couldn't spare that many soldiers--if it had that many--for the defense of the most important city.
• In order to defend Ravenna, the capital of the Western Roman Empire, the eastern empire had to send 4,000 soldiers. Again the army of Italy appears to have been woefully short on troops. 
• Honorius negotiated to get 10,000 Huns to fight Alaric. Hunnic reinforcements would hardly have been necessary if the western empire still had a decent army of its own.
• Three times Alaric besieged Rome. It would've been quite risky for him to spread his forces out along the circumference of a big city if a Roman field army were still present. The attenuated besiegers could be attacked with concentrated force. If Heather is right, and only 10,000 men deserted, there still would've been an army of 15,000 available. Apparently quite confident, Alaric faced no army worth mentioning.

The conclusion seems inescapable: The army of Stilicho, which probably had around 30,000 men prior to his fall, evaporated soon afterwards. For that to have happened, the vast bulk of the men, or most of the 30,000, had to have quit the army. I think Zosimus exaggerated only slightly, if he exaggerated at all. His figure is the most accurate we're ever likely to have.

The weakness of the WRE, following alienation of barbarians, highlights the fundamental problem--loss of citizen support for the empire. The day the barbarians quit the army was the day the army nearly ceased to exist. Few citizens would serve anymore. Alienating barbarian troops, without a good alternative source of recruits, was an awful blunder.

Roman guard.

Heather, Peter. The Fall of the Roman Empire a New History of Rome and the Barbarians. 2006