Over 10,500 customers

14,500 is the limit

My question in situations like this though, is why import customer info at all if you have it in a third party software, which I assume is ecommerce. Why not just use QB as the accounting software it really is, and import sales figures to a generic customer?

You need to manage this before you reach 14,500 and that Locks your file for customer names. You would edit and rename to Merge these. I agree with Rustler; the point is not to load the file with names you will never user again. For instance, Sales Receipt needs no name at all. Sales Receipt is used for Sold and Paid this same date. And Sales Receipt is used for "Daily Sales Summary."

Customer names are required for AR, but if you have a third party system managing this, you don't even need to use AR processes and Invoices in QB. The Summary Sales Receipt can account for these activities as Other Asset, for amounts paid on charge accounts and new sales put on charge accounts.