At my first encounter of "cei", I thought for sure that is was a loaner word from the USA word, OK or "Kay". But no, it's the future tense of the verb-noun "cael", to have. As is "chei", "cewch", and "chwech", depending upon the person or persons, and whether positive or negative.
cei and cewch are forms of cael. If they are used as negatives the they take an aspirate mutation to chei and chewch:
- [Cei di/Cewch chi] gawl i ginio. - You will have soup for lunch.
- [Chei di/Chewch chi] ddim cawl i ginio. - You won't have soup for lunch.
chwech - six. Nothing to do with cael.
I think cei is used only in sentences that use a form of the verb cael, such as "Ga i oren? Cei." There is no word in Welsh that literally means "Yes". When you want to answer a question affirmatively in Welsh, you repeat the verb that is used in the question.
Here's a good explanation of how to say "yes" in Welsh:
Look especially at sequence #4.