Tank Size

I was wondering how many Axolotls could I put in a long 20 gallon tank

You could fit 2 easy possibly 3 if you have sufficient filtering and no biting.

Thanks :happy:

Having done 3 axies in a small tank (Thanks to the Aussie summer), I'd say for the size you've got two would be best, or you're going to have to work VERY hard to keep the filtration system working, the tank clean and the water parameters optimal. If you want to go to three, get HEAPS of plants, lots of hides (PVC pipe works excellently), and be very strict about how you're feeding them to avoid "I'm feeling hungry, and you look taaaasty, You're right beside me, I'll nom you maybe" incidents (Which can happen at any size tank- My first bitten limb was in a not-overcrowded 4 ft tank AFTER their sojurn in the small tank...).
Now, I'm definitely NOT saying don't go three, but be aware of the risks and what you're going to have to do to make it the best living environment for your babies Also, you could upgrade your tank again...

One big one or two less than 20cm.

And if you are new to axolotl keeping or aquariums in general, it would be highly advisable to start with just the one.
Personally, i´m of the opinion that a 20 gallon is only adequate for a single axolotl, but even if you decide to have two in the future, starting with one and getting the hang of it is the best option by far.