Where did they keep the mules in garrison?

[Paullus Scipio]

Therefore, a smaller number of mules would be needed, possibly, say, for moving supplies from the main forts to the milecastles etc, on Hadrians wall

An interesting thought. Of course, one of the reasons for building roads was to enable the use of waggons for heavy or bulky loads.

....Indeed one might go further, and point out that the primary purpose of going to the trouble and expense of building roads is to allow wheeled traffic, which requires a hard surface. Certainly it has usually been assumed that roads were built to allow the Legions etc to move about quickly, but this is something of a myth. Neither Infantry nor Cavalry move significantly faster on roads (tactically) but roads for transport allow the army as a whole to move more quickly and easily.(strategically)
Roads are hard on animal hooves, and infantry feet (think of hob-nailed caligae or modern boots on hard surfaces) and throughout military history, Infantry and Cavalry units have preferred to march alongside roads, depending on terrain conditions.

As to wagons, they are extremely useful for carrying heavy loads - a cart could carry 2.5-3 times the load of a mule for example, and a wagon up to 5 and maybe 10 mule loads. It was this heavy load carrying ability that justified the expense of building roads.....

Large wagons were often pulled by oxen, at least in civilian life. The disadvantage militarily was that oxen were slow - able to cover a maximum of 20-25 miles only per day on flat terrain. It is unlikely that the Roman Army maintained oxen as baggage animals as a general rule, since references to them are few compared to mules - they were more likely to be requisitioned as needed.

Speaking of requisitioning...

Paul Elliott wrote:

I had always envisaged a general military round-up of civillian animals in the surrounding area ready for a campaign.

This is certainly the case, though care had to be taken in 'friendly' territory. It was not unusual for patriotic citizens to bear the expense of supplying the Army, and when the Army requisitioned supplies from civil sources, it was normal for the civilians to transport them at their own expense, using their own transport.
Requisitioning 'transport' was not limited to animals either - the Army on occasion conscripted porters as pack-animals too.
Plutarch reports (Antony 68:
"My great-grandfather used to tell how in Antony's last war the whole of the citizens of Chaeronea were put in requisition to bring down grain to the coast at Anticyra, each man carrying a certain load, and soldiers standing by to urge them on with whips."

Clearly this was an emergency, and the citizens - starving because of Antony's grain requisitions - were spared when news arrived of Antony's defeat at Actium, ending the war. Octavian allowed them to keep the grain for themselves, and thus avoid widespread famine. ( generous of him since it was theirs in the first place! )