A couple of factors determine the invoice number that the system assigns to an invoice.
When an invoice gets created, the system will attempt to use the invoice number that you have set in your Billing Settings. Every time an invoice gets created, that set invoice number gets updated sequentially for the next invoice that gets created.
If the next number that is set is already in use by another invoice, the system will find the invoice in the system with the highest number, and then work off of that number continuing forward.
Importing invoices will not adjust the set invoice number that the system will attempt to use next. However, importing invoices will introduce new invoice numbers into the system, and if those numbers are higher than other numbers in the system, then those imported invoice numbers will be the invoice number basis if the system ever determines that the next invoice number is already in use.