keep (a) count (of someone or something)

To learn and remember how many people or things are present in a given situation. Mr. Smith, will you please keep a count of the students as they board the bus? A: "And how many people entered the shop on the day of the robbery?" B: "I don't know, I wasn't keeping count." You'll need to keep count of your taxi rides and meals during the trip so that you can make an accurate expense claim when you're back.

keep (a) ˈcount (of something)

know how many there are of something: Make sure you keep count of all the phone calls you make so you can claim the money back later.