Hi @LiviaD1,
I believe there are no limitations to custom fields you can have on the custom resources. But you will have to perform analysis of which are fields required to send any marketing campaigns and other marketing activities rather than creating too many fields which is use of no use or less likely used.
Depends on number of fields you have it will affect your entire performance of ACS like loading data into ACS, query processing and other activities you do on the platform.
Thanks!
SK
Thanks, Sathees