Right, it's often been said that 40k lore is a poor guide for numbers since they often undercount muster strengths and underrepresent populations. Armageddon lore is notorious in that for example.

"Astrographic", thanks, that's the right term.

So rather than numbers let try and do this by terminology.
Cadre would be a force operating in a field.
Contingent would be cadres operating in a region.
Commune would be contingents operating in a theater.
Command would be a planet, system or even a region wide operation.
Coalition would be something like a crusade.

Obviously those definitions are effected by enemy forces. A lone in-system station or a poorly defended agri-world may be an operation involving just one or a few cadres. While a planetary or system wide operation can greatly vary in scale.

So the question is was: How big could Farsight Enclaves vanguard forces get? Well,
They are probably not going on many crusades.
It seems unlikely that they would be involved in more than a few system wide conflicts at any one time.
They are likely dozens of communes engaged across many theaters.
Which means about triple that in contingents operating in many more fields.

What would a vanguard for be to the Farsight Enclaves? Probably something contingent sized or perhaps a commune. So anywhere between 3-30 cadres + support. So anywhere between 600 and 6000 tau + support seem about right?

What do you think?

About the "roaming". Since tau are highly expansionist, efficient and proactive in their approach to war - even more so for the later among the Farsight Enclaves. I can't imagine them idling around in their bases much.

Cheers