Stock question

This won't be happening for a while, if ever, but I'd like some insight so I can think about it!

I'm considering upgrading my 10 gal to a 20+ gal eventually. Probably won't get over 20 gal, though, because of housing restrictions, blah blah blah. Anyways!

I currently have 3 zebra danio and 4 neon tetra. If I were to have 20 gallons (lightly planted, I guess - I've got java ferns, but that's all), how many more of each would they be comfortable with me putting in? I think I'd rather stick to the same two species and just increase their school size, rather than leaving them with small schools, so I'm not worried about "what else" can I get at the moment.

Thanks for the replies, everyone! I'll definately keep them in mind if/when I get a larger tank. I love the thought of a bunch of blue and orange and silver swimming around eventually. Hopefully I'll get my own place within the next year or so!
I've never heard of corydoras before. I'll definately have to look around while I've got the time to think on potential future stocks. I'm really fond of the tetras and danios, though.
Thanks again!