CORRECT is "Master of CeremonIES". This 'person' is the one who administers (for lack of a better term) the PROGRAM / AGENDA! USUALLY, there are a number of 'speakers' at any 'CEREMONY', and EACH of the 'speakers' is "Introduced"... by the 'Master of Ceremonies'...
PLURAL : ?? If you have more than ONE "M.C.", it would be MasterS of ... The only thing I can think of like that is with some of the 'AWARD' shows, where they actually have "Co-Hosts"! (NO-ONE can 'help' what 'BROADWAY', or 'HOLLYWOOD' does... Isn't there a 'Program called 'Planet Hollywood'?)
Think of it this way...
Think about any 'CEREMONY' that you plan to attend! For example, Graduation from High-School is ONE CEREMONY! There are usually a LOT of things that HAPPEN... (And, these are probably NOT in the correct ORDER!) The Class marches IN. The Principal (I'm more worried about how to spell THAT word) says a few words. I don't KNOW anymore, but there USED to be a "Holy Man' (so I don't offend anyone) who offers a 'Blessing' on the Class! The School Band usually gets a chance to 'perform! There is a 'Speech' by the 'Salutatorian'(sp?), and the 'Valedictorian'! And, Usually, there is some "Prominent" person - some Dignitary - who gives a 'motivational / congratulatory' speech/ message! Toward the END of "THE Ceremony", the Principal, with some help, passes out the Diplomas, as the Graduates walk across the stage! I know there is some more "talk", but I don't remember who does the 'talking', and THEN, the kids throw their 'past-boards' up in the air! (And then, they try to find at least ONE, so they can return the outfit they rented, without having to pay for missing parts!)
ALL of THAT, falls under ONE 'ceremony', and under ONE M.C.!!!
It's much the same in the Military - Graduation from 'BOOT-CAMP', or 'AIT', etc! Graduation from COLLEGE, or UNIVERSITY!
Hope this helps!
***** If a lady is the host of the program, do we still refer to her as the master of cermony/ies? How about if the hosts are a lady and a gentleman?
* If a 'lady' is (YOUR WORDS) "the host", then YES, SHE is the Master of Ceremonies!
* Again, (YOUR WORDS), if you are going to have "the hosts are a lady and a gentleman?"... then you have CO-HOSTS!!!
If it makes it any easier, SOME programs will say: " Your HOST, and MASTER of CEREMONIES..."
Again, I Hope this helps!