If need be is an idiom which means if the need manifests. It became an idiom when conditional phrases (beginning with if) stopped requiring the subjunctive. Now we say “if it is right”, not “if it be right”. It is used most often as a modifying phrase and does not require commas unless needed for clarity, which may often be the case.
Most dictionaries, if they list it as an idiom at all, list the singular. Therefore, in writing the singular form is used almost 10:1. But if needs be could easily be interpreted as if the needs manifest. And it is our experience that in informal or spoken language, the singular and plural form of this idiom are used almost interchangeably depending on the user’s personal preference.