Stocking Advice For 450 Litre Tank

would I be able to put 50 cardinals in at one go to start a single species community tank

I understand from your other posts that you are currently running this tank empty. Now is a good time to buy some ammonia and start cycling  If you don't cycle it first, a lot of those cardinals are likely to die; they are not the hardiest of fish, though they are reputedly hardier than neons.

If you cycle it and you want to risk it, you could get more cardinals than that since you say you want a single species tank. Those 50 you asked about would be around a fifth of the number you can have if you want to push the stocking to the limit. The idea behind fishless cycling is that you grow more bacteria than would be needed by a tankful of fish so you can stock to around 75% or 80% of the total fish as soon as the cycle is finished.