I see lumps on the roof of my mouth - could they be cancers?

Probably not. Painless firm lumps could be palatal tori. Which are painless bony lumps that for some reason form in certain peoples mouths. See your dentist or an ENT for an accurate diagnosis.
No. If you don't smoke, use smokeless tobacco or drink alcohol excessively, then chances are those bumps are not cancers. Oral cavity cancers are usually single, ulcerative, painful and unattractive.