How many days do I have to leave Thailand after cancelling my Non-ED visa?

That depends on your last day on the termination letter, not the current stamp or when you cancel your extension.

You cannot stay longer than that day.

You need a termination letter from your school to cancel your extension based on studying, not the first 90-day stamp from the Non-ED visa. That letter has your last date.

When you apply for the cancellation in advance, the last day you can stay will be that date.

When you apply for it on the last day, you must leave within that day, on or before 23:59.