That would depend on a lot of circumstances. When using a web interface, the problem is how long it takes in time, not how many records you have. (It will execute for XX seconds before giving up.) So one batch may work at 10k, but another might fail at 5k because there was something that took longer to process in that smaller batch.
Personally if you're using something that big, it might be worth using the API to create the accounts.
https://invisioncommunity.com/developers/rest-api?endpoint=core/members/POSTindex
If you don't want to do that, start small... 1k and see if it works. If it does, increase it and try a bigger batch. Rinse and repeat until you find what works for you.