Depends. You can have a bunch of different types of partners - long term, committed partners, casual but committed partners, strictly sexual partners and uncommitted, strictly sexual and committed... I honestly could go on and on.
My point with this is, it depends on the kind of partner you mean. And also what each person can handle. Some people can't imagine themselves poly because they couldn't handle more than one person, and some people are perfectly comfortable living with and committing to 10+ people for the rest of their lives. It honestly depends on the person.
I have two partners, both committed, and right now, that's a good number for me. I think I'd probably max out my "I can handle these people" threshold at 3 or 4 committed partners (and casual or strictly sexual, not a problem to handle, so I couldn't give you a number on how many of those is too many for me). I'm a very introverted person, and also not good with those romantic emotion things, or even those emotion things at all, and even having two sometimes can be rough.