If my very novice opinion counts; I would say that 'too many' is when you start to feel over-stretched. The capacity for love may be absolutely infinite. But the ability to spend adiquate time and affection on each lover is something that will vary from person to person and lifestyle to lifestyle.
My mother would say that anything that is supposed to be a pleasure should STAY a pleasure.
At the point at which is becomes a stress, it's no longer doing what it should be doing.
Not to say that at the first sign of stress one ought to bow out of loving relationships. But if I were you I'd look not just as 'how much love I can give' but 'can I maintain these relationships in high quality with the time, strength, energy, and responsiblities that I have'.
Hope I didn't sound like a downer there. LOL
With all my heart I will love and not fail,
With all my soul I will fly and not fall.