How Many Would Be Needed?

Good question.  The Protev has a crew complement of 500, so it couldn't be any more than that.

If we presume the Arcova's crew (75) is the bare minimum to operate the craft, then 425 represents the "colonizing" crew.  Colonizing requires 90 antimatter, and total antimatter capacity is 225, so my best guess is 90*425/225 = 170 people.

I'm presuming this is just the initial force, and additional forces would arrive shortly afterwards (this would be abstract, in the same way that the game references ship supply chains that don't appear in-game.  We just hand-wave it and pretend it's happening in the background).

 I'm thinking that it would depend on planet type seeing that all inhabited planets have a militia and therefore a ground side populace with Terran planets requiring the most ground occupation forces and Asteroids and Dead Asteroids requiring the least.

Also,that's just for the TEC,the Vasari Migrator carries an on board population of about 1,600 personnel most of them probably being occupational units and the Advent Missionary Vessel carries 1,100 personnel.

True, I was using TEC as a baseline, but I'm more or less making a guess on the subject anyways.  The Vasari number is largely irrelevant since the manual indicates all Vasari ships carry an unspecified number of civilians in addition to military personnel.  So really we're just comparing the Advent and TEC varieties.

 True, alright the Vasari are out of the equation, who do you would win a ground battle, TEC or Advent?

The implication I get from the game is that ground combat is essentially super-charged version of trench warfare where the defender always wins, and the only way to get a foothold is to blast them out with orbital bombardment.

Well...judging by all the ridiculous psi-tech powers the Advent have, I would think they'd have a huge advantage...I don't care what defenses you have, you see giant white hands crushing your comrades as others get flung back at half mach speed, you'll be shatting yourself and high-tailing it outta there....

Of course, how well Advent can use their abilities on the ground is a mystery...my closest guess is it works sort of like adepts in mass effect...as Darvin has mentioned though, it seems to not really matter as whatever they have is outclassed by orbital bombardment...