You have identified something that is often an issue in poly relationships.
One of the mantras about poly is that love isn't a zero-sum game - that a poly person believes that they have an infinite capacity for love. The problem is that when it comes down to practicalities there are two "zero-sum games" - money and time. It'd be nice if we had an unlimited supply of both, but 99% of us don't have unlimited money and nobody has unlimited time.
So at some point you reach what I and others call "poly saturation" - you are using either all your time or money (or both) on the relationships. This either means that you have to have a "no more relationships" or you have to spread your limited resources thinner. That can be an emotive issue, and one that needs to be worked on with everyone involved, so that expectations are managed, and synergies found.
I don't think that there is a "one size fits all" solution to this, either - it has so much to do with your own situations, boundaries and needs, and those of your partners. But if it is not dealt with, then it can become a major show-stopper issue.
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