While "local maximas" is wrong, I think "a local maxima" is a valid way to say "a member of the set of local maxima" regardless of the number of elements in the set. It could even be a singleton.
No, a member of the set of local maxima is a a local maximum, just like a member of the set of people is a person, because it is a definite singular.
The plural is also used for indefinite number, so “the set of local maxima” remains correct even if the set has cardinality 1, but a member of the set has definite singular number irrespective of the cardinality of the set.
You can't have one local maxima, it would be the global maxima. So by saying local maxima you're assuming the local is just a piece of a larger whole, even if that global state is otherwise undefined.
No, you can’t have one local maxima, or one global maxima, because it’s plural. You can have one local or global maximum, or two (or more) local or global maxima.
