Just a couple nuances to consider.
I personally think if you are an “unhooked” candidate applying to a general admission college, or to something equivalent like general admissions to the Arts and Science school at a university, and it doesn’t have rolling admissions, you typically do not NEED to apply early, understanding if you want scholarship consideration there may be a deadline for that.
The cases where slots might “fill up” are typically things like rolling admissions colleges, restricted majors, and specialty schools which admit first-years. Notably, in practice most of those are exceptions in some way to the “restricted” or “single choice” early action rules, and similarly usually are not barred by Early Decision. Meaning those schools are trying not to do anything to really prejudice you, although if your only early options are ED for a restricted major/school or REA/SCEA, you may have to decide which is your first choice–but that is obviously considered a “fair” choice.
Some people will also suggest in some cases, even at general admissions colleges/schools there can be de facto “slots” for things like people from a particularly small college student state, people who intend a particularly unpopular major whose department might be lobbying for a minimum number of admits, people who play a particular instrument, or so on. I personally think these sorts of informal “hooks” might exist sometimes, but I think it would logically be very rare, would not likely be a strong factor in admissions for non-binding early applicants (since there is not much yield advantage), and generally is probably something most people can kinda know doesn’t apply to them (like if you are an intended Biology or Econ major from New York or California with no special performing arts talent, you are probably safe to ignore this issue).
But this doesn’t mean it is BAD to apply more places early. Still, there are some obvious temptations, like maybe if things go well with one rolling admission place, you can cut out some other “likely”/“safety” applications entirely. Or, of course, if you get into your favorite ED/SCEA/REA, that may cut out many applications. And so on.
Also, this is very personal, but spacing your applications out a bit more can potentially allow you to give them each some real thought and care.
And our experienced college counselors give advice reflecting that. They want you to have a thoughtful plan, to not miss scholarship deadlines, to make sure you don’t do anything too prejudicial if you are targeting restricted majors/schools, and so on. But other than that, they also think you can have “enough” early applications if together they, say, allow you to get an early answer from one or two promising likelies, and possibly your favorite.