Oddly enough, I don't remember seeing this question asked before. And I gotta admit that I can't interpret a good answer for you from the rulebook, as the rules are mum on the matter. However of course, if the shape of the token is any indication, then the max would be three as each token is shaped as 1/3 of a disc/circle. Thusly three are meant to be joined together when placed on a tile, as the illustration on page 19 implies.
I don't recall exactly which set(s) they are from, but some locations specifically state that outposts/colonies/starbases require an additional number of influence tokens.
This, to me, has always implied that the number of influence tokens that can be in a single location is unlimited. Strategically, it is rarely a good choice to pursue an installation in these spots, but it could be done if absolutely necessary.
I fall on the side of the discussion that claims that there is no limit. Although there are not many reasons to have more than 3 or 4 in a hex. Your Actions are usually better used for other things than simply piling up Influence Tokens. Depending on the situation, of course.