>How many space marines are there?
Their total numbers are estimated at a million.
>Are chapters really capped at 1000?
According to the Codex Astartes, yes. Some non-codex-compliant chapters are larger, but they avoid concentrating their forces so as not to alarm Imperial authorities. It may have been retconned, but at one point the Black Templars were estimated number almost 6,000, but you'd never see more than a few hundred of them in a single crusade, and their command structure didn't put all 6,000 under any one person's control.
>I don't care how badass you are, 1000 dudes and a handful of vehicles can't do much.
Depends on how you use them. Remember: Astartes are shock troops, not garrison forces. They don't go in to hold territory. They hit command and control systems, wreck logistical infrastructure, sevre supply lines, capture high value targets, etc.
Here's how an invasion of present day earth would go:
Day 1: The Strike Cruiser Lament of Korrath arrives in orbit carrying the III company of the Ice Wraiths Chapter of the Adeptus Astartes. The 3rd is supplemented by one squad of veterans in Terminator armor, one squad of Red Wendigos (Death Company), two squads of scouts, and one dreadnought. They demand the immediate and unconditional surrender of the planet in the name of the Emperor. This demand is unwisely refused.
1d4chan.org/wiki/Ice_Wraiths
1d4chan.org/wiki/Ice_Wraiths_Writefaggotry
Day 2 - 10: Orbital bombardment is used to reduce local political centers and military bases. Washington DC, Moscow, Beijing, London, Brussels, Paris, Berlin, and Tokyo are all wrecked. Civilian leaders of most major nations are dead or in hiding. Military command and control survives in US and Russia due to old cold war infrastructure and protocols, but their forces are scattered and dare not assemble en masse without inviting more strikes.
cont.