A quick demo showcases that there's not much of a limit to the number:
Warning, the above creates a 5GB file and will take a few moments to run the first time you compile and execute it, as it's allocating disk space and all for that massive text file, before writing those two data entries.
A LONG variable only allows us to access 2,147,483,647 elements before we overflow, and as you can plainly see, we're putting data to records beyond that. QB64's internals, for records, is probably limited by _INTEGER64 values -- meaning we can have a maximum of 9,223,372,036,854,775,807 records. If your drive can hold more than that, then you don't need to be coding in QB64! Instead, you need to send me a few million dollars, and I'll become your personal coding machine, forever more! :D