Answer :

It depends how you use it. Neighbors without an apostrophe is just referring to two or more neighbors as a subject, but if the apostrophe is used before the s, one neighbor is in possession of something. If you are trying to say that more than one neighbor owns something, you would put the apostrophe after the s.
the word neighbors can have apostrophe depending on the way it's used. when it's used as an ownership, that's when it has apostrophe but when you're discribing more than one person, that when it's just "S"

for example: my neighbor's cat jumped across the fence OR i have 2 neighbors living across from me. see the difference???