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.
Examples
He wants to leave open the option of listing the division on the stock market, if need be. [DW]
High school principal Chris Hudson said their teachers will now not just hide in the case of an emergency, but be prepared to fight back if need be. [Nevada Daily Mail]
As Mr Lambert’s solution suggests, our manly real estate – recognised in this instance as starting and ending at the forearm – should be left as clear as possible, the better to decorate with a decent watch, and if needs be, a little male jewellery. [The Telegraph]
“Expect more vehicles to be fitted with anti-collision systems that automatically detect the cars around you and move the car into another lane if needs be,” according to one tech site’s list of predictions for 2015. [Forbes]