Or at least, it wouldn't take hundreds of them to overwhelm a squad of Terminators. That said, Genestealers wouldn't be completely overwhelmed by Terminators. There's no way to properly balance the game around 'Lore Marines,' unless you want to have ridiculous army size discrepancies, like a couple dozen Marines versus several hundred Orks. A single marine, in the lore, is supposed to be more destructive and powerful than a squad of 30+ Imperial Guardsmen, whereas in the game, a single marine is worth maybe 2-3 IG, and probably not even that. Space Marines are far more powerful in lore than they could ever be in a balanced war game. Though this sub generally discourages meta answers, this is really the discrepancy that OP is encountering, and as such, a meta answer is really the only one we can possibly give. Once disabled, the Marine inside is essentially a can of food, waiting to be opened, because even his prodigious strength can't move the Terminator armour without its internal systems. In close quarters, with no maneuverability, even a handful of Genestealers has an advantage over lone Terminators because they can circle around and strike at vulnerable cabling, vents and servo's of the Terminator armour. So are the Dark Angels (who featured in one version of Space Hulk). So how could this happen? Well, first, Blood Angels are, frankly, unstable, and prone to poor decision making. That does not, of course, invalidate the fact that an entire company of Blood Angels were wiped out by Genestealers. This is because the Space Hulk rules actually pre-date Terminators having active shields, represented in normal 40k by Invulnerable Saves. Genestealers have adamantium laced claws which allow them to carve through just about anything, but Space Hulk fails to account for the shield generators.

However, Space Hulk also can't be taken at face value. They are an infiltration and infestation organism, and appearing in such numbers before even reaching a planet is counter to their design. While Genestealers do vastly outnumber Terminators in almost every situation, they don't typically infest Space Hulks in the thousands.

Warhammer 40k terminator armor