كم How many/How much: used to ask about something to be counted, usually. بكم : How much does.......cost? (usually used to ask about cost/price/rent..etc.).

كم like how many? the answer usually in number, like 5 cars. بكم like how much is it? the answer usually in money, like 5$.

Difference is the letter بــ Which as a prefix would be like in- or en- كم is quantity بكم is in-quantity (=in total)