How automation agencies work with their clients? For AI agency owners and workers

I'm in the process of launching a new agency focused on AI automation, and I have a few questions for those experienced in this field. Specifically, I'm curious about account and contact number management. Should I be managing all the accounts and new contact numbers directly, or is it standard practice for the client to create their own account and then grant me access? I encountered a hurdle when I attempted to add another contact number to my account; a notification appeared stating I've reached the limit. Any advice or guidance on this matter would be greatly appreciated.