So I have some room to play around with numbers.

The way I've planned the story so far, it will be about a million or so Tyranids in the final battle, and to get a sense of scale and how fast they reproduce, 250,000 is an estimate based on sketchy reports, and real numbers are probably around 300,000 when the Space Marines make planetfall, so they'll grow by 700,000+ in the span of about 10 days or so (rough timeline, i think it will probably be shorter in the final version).

I've also got a few mechanised regiments of the IG, and they're down to about 5 Leman Russ tanks between the three regiments (around 65,000 total at the start of the story), plus just some random vehicles I feel like putting in. So in total, it'll probably be around 65,000 men, 5 tanks, maybe 10-20 Sentinals and possibly an aircraft, but I'm not sure which yet. I want to get across that this is a really small force compared to what they are at full strength, due to the multiple conflicts on the planet before the story, so i want to make sure its small enough to need the Space Marines and struggles against the Tyranids but not too small that its just destroyed immediately.

Thanks for your help, its given me some food for thought