Hello Nwag,
When you perform a NAT translation, the prefix that you define is not added to the translation, it simply narrows down to source and destination IP addresses, the prefix or mask is used more details to static network translations and to define the ranges for the traffic that you want to translate.
Anyway your ISP controls the IP address that are routed to your router, so even if you translate the traffic to an IP address it does not guarantee that you will get that traffic back. If you need more addresses you will need to purchase them.
Hopes this answer your inquiries.
Regards,
Alex Sanchez
CCIE R&S #37454