I'm planning to pursue my STEM degree abroad.How many universities can I apply at once?

You can technically apply to an unlimited amount of universities, but I don’t think that’s what you wanted for an answer.

Well, it depends. It does not make sense to apply to more than 20 schools, as the amount of work increases. Also, most application portals don’t allow you to apply to more than 20. For example, the CommonApp limits how many colleges you can submit an application for to 20. I am not sure about the Coalition or Universal College applications, though I would assume it to be the same.

Have a good balance of schools in your list. After all, you want to be accepted into at least a few of the schools you spend months and months completing applications for, right?

Have a balance of safety, target, and reach schools in your list. Consider if the college(s) consider demonstrated interest. Attend college information sessions to find out if your goals match with the college and if the college vibe matches with you. There are a myriad of circumstances to consider.

tl;dr: A good rule of thumb is no more than 20 because at that point it’s basically wasted effort. A few QUALITY applications is much, much better than a multitude of less quality applications.