It will depend on the accent of the speaker. I'm British and pronounce it with three syllables - Ko Ren Ri. Other speakers might well pronounce it with four or might use the one you hear. Check in both BE and AE dictionaries to see if they differ. Good dictionaries tend to list alternative pronunciations though.

As far as I know, American pronunciation is just "corn-nary". It's one of those words that just has a weird pronunciation (like for example, the word "colonel"). I hope this sort of helps! 😅

My advice is to pronounce it with four syllables because that is definitely correct, and definitely common in the US. In the United States the pronunciation varies. Sometimes I hear three syllables, sometimes I hear a gentle hint of the "-ron-" sometimes I hear four. My honest guess is that people THINK they are saying four syllables, but don't move their tongue far enough to make it audible. I THINK I say four. In my HEAD I am saying four. My advice is to pronounce it with four because that is definitely correct, definitely common in the United States, and will always be understood. The vowel sound of the first "o" varies; sometimes the "cor-" sounds like "car," sometimes like "core." Checking the American Heritage Dictionary, it only lists two pronunciations, both with four syllables--one with the "car-" vowel sound, one with the "core-" sound. The second syllable is a schwa, which is often light and hard to hear.

Pronounced correctly the word has four syllables. There are many words that people often pronounce incorrectly, and this is one of them. Everybody here has his own pet peeve about some commonly mispronounced word. The one that burns me up the most is "insurance". Many people put the accent on the first symbol and that really grates on me. You can find websites that display long lists of commonly mispronounced words. Since you are learning English, my advice is to try to stick with correct pronunciation. Over time, a correct pronunciation can change. For example, in the words that start with "kn" like "knife", "knead", "knight", the "k" used to be pronounced but today it is silent. As far as I know, "coronary" still has four syllables. That's how I say it, but there is no linguistic police force in the U.S. so people can mangle it as they like.

I pronounced it with ren in between