I am currently using Quickbooks Enterprise. My customer file is large, approximately 10000 customers. Will a customer file this large import into Quickbooks Online?

You have to contact MMB to convert their data. But if they can't help you, you may consider purchasing another 3rd party conversion service. You also need to purchase another 3rd party service to convert data with attachments/multicurrency.

https://movemybooks.co.uk/move-to-quickbooks/limitations-of-service-quickbooks/

Welcome to the community, @AJS1986.

Let me chime in and shed some information about importing.

When importing data, make sure your Total Targets is less than 750,000. If the file exceeds the target limit (750,000) it can't be converted to QuickBooks Online. The only option is, you can only import lists and balances into QuickBooks Online or start fresh. Press Ctrl + 1 on your keyboard to check. 

I suggest giving this article a visit for your reference: Move your QuickBooks Desktop file to QuickBooks Online.

Additionally, here's an article that you can use to know what will happen after migrating your data: QuickBooks Desktop discontinuation support hub.

Please feel free to leave a comment below if you have any other questions. Help is just a poll away.