Pretty sure it's 5 sirens you can have in total, but I have no idea. I have seen fivem videos and been in servers where there are way more sirens in game for specific vehicles, even like 1 vehicle out of an entire pack has a different siren than all the others, (i'm guessing to emulate realism based off of whatever department). How can I get more than just the regular amount of sirens in my game, is there a way? Or is there not? I have heard of luxart vehicle control being able to do these kinds of things but its only for fivem (which stinks). Any help?

You theoretically could, yes. Doug1637 has a great YouTube video that goes into depth regarding that, as well as there are several forums posts already covering this. I won't repeat the tutorials because of how lengthy it can get.