Modus operandi is singular in both Latin and English. The plural is modi operandi, and, judging from this Ngram, I would advise against modus operandis.
Since there seems to be some confusion over why only modus changes form in the plural, but never operandi, I'll explain that too.
Modus is the most important word here, quite clearly meaning mode. It's a noun and nouns are simple. The plural is modi.
Operandi is rather more complex. We may translate it as of operation, but that's not very precise: mode of operation would be modus operationis. More exactly, operandi means of operating. It's a gerund, which can't be pluralized either in Latin or in English. However many times you've gone fishing, you'll still say that you like fishing, never that you like fishings. It's the same in Latin.
The confusion arises because the Latin gerund shares its singular forms with the gerundive, which can be pluralized. We know that modus operandi does not use the gerundive because then it would mean mode of something to be performed, which is rather nonsensical. So, I'd strongly advise against using modi operandorum, since I presume you don't mean modes of things to be performed - nor am I particularly sure what anyone might mean by it.
You might say that, if people used modus operandorum often enough, then using it would be fair game. You'd be right. Luckily for us, however, Ngrams doesn't have a single instance of it.