If it gives you any indication of how many fish you can have in your 40...
My 40breeder, when fully stocked, will contain...
20 ember tetras (3/4" long)
2 German Blue Rams (2.5-3" long)
1 chocolate gourami (2-.25" long)
4 otos (1.5" long)
3 Stiphodon gobies (2" long but very slender)
If your fish are small, you can put more in than you might think.