Quark is another coin that does not use 8, but rather 5 zeros beyond the decimal.
The reasoning may be arbitrary. However, a 64 bit integer can hold a maximum value of 18446744073709551616.
If there are 8 zeros allowed, then the max number of coins can be:
184,467,440,737.09551616 (184 billion)
If there are 6 zeros allowed then the max number of coins can be:
18,446,744,073,709.551616 (18 trillion)