Please help. Why am I limited to having only 100 customers in my books?

I can definitely help you add more customers in QuickBooks Online (QBO), accounts551.

There's no actual limit with regards to how many customers you can add in QBO. The current list is maybe affecting the QuickBooks performance, that's why you're unable to add a new one.

The more data you have in QBO, the longer it may take for the program to open pages. Check out these articles for more details: Why is my QuickBooks Online slow?

Let me also share this article as guidance about managing customer list: Add Customers. It let's you add, edit, delete, or merge customers.

I'm still willing to answer other questions you might raise about handling your account. Take care and have a great day!