Where was the Roman Army in AD408?

[Nathan Ross]

because Italy didn't need Limitanei, that they were forced to garrison the field army regiments in walled towns as a deterrent against Alaric?

Yes, that could be right. Keeping a large force billeted in one place for lengthy periods when there isn't a campaign imminent would have been tricky, so dividing the army among various billeting cities would be probable - plus there's the mention of 'troops both horse and foot, which were in the different towns' (above)...

I think the Goths remained mostly in Etruria, when they weren't besieging Rome or menacing Ravenna, and Rutilius Namatianus in de reditu suggests that they trashed the countryside pretty thoroughly - walled cities like Luna (which Namatianus mentions) could presumably have resisted them with only a light garrison.

Still, it would only have taken a decent commander to draw together the scattered units and lead them decisively against Alaric for things to have turned out differently...

I'd imagine it had at least the Protectores and the Schola stationed there

I doubt that - there seems to have been no military force in Rome at all. The scholae and protectores would have accompanied the emperor and his court, which had departed the city for Ravenna and Ticininum in early summer 408.

Milan was important, and we've already mentioned military depots and towns such as Ticinum or Aquileia. Those three alone, assuming Ravenna's number was the average, eat up 12,000 men. Assuming the same unit composition too, that's 18 of Stilicho's 30 regiments.

He had 37 if we include the cavalry. But yes, depots and other cities could have accounted for a large-ish force - but to leave such a force in garrison while an enemy was devastating your country, and to have to call troops from elsewhere and even hire mercenaries, still implies that for some reason the army was not in any sort of effective state or mood.