The plural of guess is guesses. You could say "I have a guess" or "I have some guesses" but it is more common to say "I have an idea (or some ideas)" or "I have a hunch". Angel's suggestion that you could say "I have some suspicions" is correct but this word implies that a crime or bad deed has been done. "I have assumptions" means something completely different so don't use this word until you are sure about its meaning.
I don't know where I lost my keys but I have a guess.
I have no idea where I left my wallet.
I have a hunch that I left my keys at my sister's house.
I have some suspicions that Michael ate my lunch.