All-numbers would be worse not better? That's only 10^7, even if they kept the first one 1-9 and did 26^7,they'd have billions, seems like the obvious solution,

All-numbers would be worse not better? That's only 10^7, even if they kept the first one 1-9 and did 26^7,they'd have billions, seems like the obvious solution, but I take it there must be some limitations that make it hard to go there.