This is a spoiler for all those that reach end-game (Tier 8 in Prestige)

To maximize your input, you have to bypass the Input Bug .
To do that enter a number (e.g. 9) and enter it up to your desired maximum (e.g. 18 * 10 - 10000000000000000)
You'll end up with something that looks like 9.999999K (if you enter 7*9's )
Remove the period (e.g. 9999999K)
THEN remove the K, and it'll auto correct the number.

The number you want is this:
Lv9,223,372,036,854,775,807
It is 19 digits long, and cannot go higher.