If you just want neons you could fit a maximum of 30-40 depending on the exact length and width of the tank (by the way, whta is the length, width and height of the tank?), but you'd definately need to fishless cycle the tank with pure ammonia first before you put any fish into it (check out the links i gave you). But i wouldn't really advise doing such a thing because;
a. Neons in general aren't very hardy fish, they can't cope well with unstable tank conditions that newly set up tanks so often suffer, and they often die at the first sign of ammonia, nitrites or excessive nitrates over 40. A lot of people (including me) only really advise them for tanks at least 6months mature because only at this stage is the tank safe enough and established enough to keep such fish with minimal risk of the tank encountering water quality problems etc.
b. Such a large amount of tetras would instantly stock the tank to its limits. If neons are the only fish that interest you, then thats fine, but a lot of people would want more than neons in their tank as although they are very attractive fish, they are not the most interesting of fish at times.
c. The more fish you buy at once the greater the risk of them bringing in deseases undetected because of the more fish you have to keep an eye on. Noticing whitespot symptoms for example on a single neon or two amoungst 40 odd of them would be very difficult- the quicker you ID and treat deseases and things the better, but doing such a thing on a giant shoal of neons would be very difficult etc.
Personally, if you like neon tetras i'd only go for 6-10 of them once the tank is established enough so you still have plenty of space left in the tank for stocking other fish and critters
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