"Bread costs five coins."
Translation:Panis quinque nummis constat.
Both are singular.
panis is nominative, it is used as the subject of the sentence, the thing 'performing' the verb.
Like in this sentence, the bread is what 'costs'.
panem is accusative, it is used as the direct object, the thing the action is directly done to.
Marcus panem habet -> Marcus has the bread.