As a general rule, adult axies need 1 foot of tank width each, this formula says a 6 foot tank could comfortably house 6 axies.
But you can also go on volume - 10gal/40L per axie. This says you could have 17 axies in your 6 foot tank - which is just silly.
Neither method is perfect, and there is no perfect answer.
You have think sensibly, I would say more than 6 in a 6 foot tank, but not as many as 17. I would probably go for 8 or 9. ( or 7 in a 5 ft tank)
And it's not just space - it's environment too. A well planned tank can have more axies than a 'chuck some plants and a cave in there'. You should have as many hides as axies and that takes up space too.
You also have to consider identifying parents if they breed, whether you can afford to feed them all, the cost of running the tank (electricity, water & treatments etc), and whether your floor will support a tank that size (6 ft tanks weighs about 700kg).