How do you count beats in music?

In music you have a time signature, which is the number of beats in a bar

The time signature for most music and certainly most popular music is 4/4, theres 4 beats in each bar

Listen to Stuck on a puzzle by alex turner, a very clear 4 beat repeating riff on drums

Then you can have 3/4 which is 3 beats per bar, for example Sing It again by Beck

But to actually count, drums/percussion is a good place to start, or try to tap your foot to the regular beat and count how many you do in a repeating rhythm

to be honest it will come with practice and getting an ear for it

Understand what a measure is. Music is divided into measures, which are designated by a vertical line. Notes in music are named based on how much time they take up within a measure. Think of the measure as a pie that can be cut into quarters, halves, eighths or a combination of different notes