Going back to IG structure, how many men are there in an infantry regiment?

Tanith First and Only had about 2000 men at full strength, but the Valhallan 597th only has 1000 soldiers I believe.

I know the 596th was formed out of the decimated remains of two other regiments, which would account for its initially short numbers, but they don't seem to increase that much over the years (then again, Valhalla does support a number of regiments so new recruits would be spread out among all of them).

Is it just variable depending how many people they can enlist/conscript at the time or is there an IG version of the Codex Astartes?