I've seen the phrase in both wordings E pluribus unum and Ex pluribus unum. Which one is correct?
See my follow-up question for the double meaning of this phrase.
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Both ex and e are correct before consonants, with ex more common (confirmed by Lewis and Short), while ex is of course required before vowels. In his Confessions, St Augustine wrote (IV.8) "flagrare animos et ex pluribus unum facere". As a motto of the USA, I believe it is always "e pluribus unum".
The general rule for the use of e and ex as prepositions can be found in Latin grammars like Gildersleeve's:
Ē is used before consonants only, ex before both vowels and consonants. (§417.6)
Lewis and Short write that ex is still more common than e in front of consonants, but that some forms tend to use e:
ex or ē (ex always before vowels, and elsewh. more freq. than e; e. g. in Cic. Rep. e occurs 19 times, but ex 61 times, before consonants—but no rule can be given for the usage; cf., e. g., ex and e together: qui ex corporum vinculis tamquam e carcere evolaverunt, Cic. Rep. 6, 14. But certain expressions have almost constantly the same form, as ex parte, ex sententia, ex senatus consulto, ex lege, ex tempore, etc.; but e regione, e re nata, e vestigio, e medio, and e republica used adverbially. (source)
paulatim singula uires deperdunt proprias, color est e pluribus unus, nec totus uiridis, quia lactea frusta repugnant, nec de lacte nitens, quia tot uariatur ab herbis. (Appendix Vergiliana, Moretum 1, 101–4)
So both are correct, and in Classical Latin at least, ex is more common.