Some acronyms can be naturally pluralized. For example, consider the acronym RTP which stands for regional tax policy -- the plural is regional tax policies and thus the acronym can be written as RTPs.
What about for words whose plural does not use an "s"? For example, consider the acronym NE which stands for Nash equilibrium -- the plural is Nash equilibria, so writing NEs doesn't make any sense. Should one accept the ambiguity and write NE for both Nash equilibrium and Nash equilibria?