This post explain, why BPD have few partner at same time.

How many favorite people have BPD sufferers had at one time?

It varies.. depending on mood I suppose. Often I only need one, although this can become a bit intense for the both of us and I find I’m more stable with two.. so I tend to spread out the love a little more. If one triggers me I can just forget about them for a while and focus on the other one til the trigger wears off. Three or more becomes harder to manage as favourite people stir up a lot of emotional stimulation.. it’s easier to run out of the feels when there’s more people (maybe it’s because I’m at introvert, not sure.. but spending time feeling infatuated or on cloud 9 with people can take up a lot of energy!)

The other thing is, you can have two favourite people but the one you’re with in that moment will be your most favourite. This can swap easily if you see both in one day and the latter presents a more caring connection for you… and FLIP “who was I kidding I love this person WAY more!”.. it’s quite childlike. Plus, object constancy makes it hard to remember the other as fondly if you’re being pumped full of endorphins from the current person.. you just kind of idolise that one and think they’re your whole world for that period of time.. until they trigger you anway. If one triggers me, the other instantly becomes top dog. Though this swapping can happen so frequently neither would be any the wiser.

Answer to How many favorite people have BPD sufferers had at one time? - Quora
It varies.. depending on mood I suppose. Often I only need one, although this can become a bit intense for the both of us and I find I'm more stable with t...