3 seems pretty reasonable to me, assuming you start the lessons much earlier in schooling than we currently do now. Perhaps not mandatory, but I think requiring 2 and having the option for more is reasonable enough. There are plenty of countries that begin English lessons in what would be elementary schools, then add a second European language in middle school alongside continued English classes, and have the option to do a 3 language for students who are interested/would need them for their academic plans.

Of course, if it was just two years of four different languages, that would be a waste of money, IMO. If kids started doing Spanish in 4th grade and were expected to keep that up through high school graduation, and could add German or Russian or something in middle school, it seems reasonable enough to me. You won’t be cranking out kids fluent in several languages that way, but I would expect you could get much better results than we currently do in the first foreign language, plus give them a decent foundation in the second, should they need it/decide to continue learning after 6 years of classes.