You haven't learned MSFT's m.o. yet... this kind of pop-up only shows up when you're absolutely certain about something, like removing files. In those cases they ask for confirmation endlessly and needlessly.
When they're trying to collect money, they go ahead and quickly guess with all the defaults as they see fit. I'm surprised the default isn't 1000 seats tbh. After all, you're certain to be a huge success now that you've chosen them as a vendor.
;-)
New to Microsoft ? You get more help from someone in the street than from Microsoft.
Did you read the TOS before buying ? Microsoft has very good lawyers.