"Ga i beth os gwelwch chi'n dda?"
Translation:May I have some, please?
peth is a word with a number of meanings and uses, and this is one where it means 'some, a little, a little bit'.
rhai also means 'some' but, on its own, usually only when referring to something already mentioned:
- Dyma fy esgidau fi, ble mae'ch rhai chi - Here are my shoes, where are yours/your ones?
- Dw i'n hoffi'r losin 'na, ga i rai, plîs? - I like those sweets, may I have some, please?
(Remember the mutation after ga i... - rhai -> rai)