"How many foods do you buy?"
Translation:Quot cibos emitis?
Cibus is singular nominative, subject, the agent of a verb, as in 'cibus mangnus erat', the food was large.
However, cibos is the object plural, as in 'cibos multos habemus', we have many foods.
Remember the general pattern for these declension of masculine nouns:
Singular: cibus, cibum, cibi, cibo, cibo; plural: cibi, cibos, ciborum, cibis, cibis.