As many as can handle him, that he in turn can handle. Multiple relationships are usually not a matter of nobility, but access, intention, and attention.
Maintaining all three over multiple relationships of the same type and energy level can be exceedingly difficult, even disregarding social/cultural mores on the subject.
Put another way, if the "noble man" in question cannot be a 100% participant in a single relationship, then adding more is generally a terrible idea.
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