I mean, to be fair I seriously doubt anyone is legitimately going to purchase that many cycles, but also I get this error message when I click buy, the page doesn’t seem to crash: Maximum amount is 1,000,000 Cycles. ($10,000 US worth of cycles… 999999999999 cycles is actually $10 billion US… so there is almost no chance anyone will ever enter that number legitimately anyway).
When you get to around 16 it rounds all the 9s up by 1 cycle and at the end of the huge number adds 20, so that now each 9 you enter is replaced with a 0