[@Shoryu]
The description for the coins stated that they wouldn't be of much use outside the character creation, so it's recommended you use them all.
There's separate rules for becoming a monster girl in the Patron link.

Edit: For clarification, to become a monster girl, you first must either get the Patron benefit Magical Overload or use a Black Coin and ask for a lot of power. Coins don't affect it, although monster girl allies get [s]less[/s] no coins if you roll the Allies perk.