While practicing pranayama (prana = vital life force energy, yama = control or expansion of) it's useful to count the breaths to sustain a rhythm and also keep track of how long and how many breaths have taken place.
One way is to count by chanting "Om" and a number internally.
For example: "Om 1, Om 2, Om 3"
Another way I learned recently, is to use one hand and count with you thumb on the creases of your fingers as illustrated below. This method can be used while internally chanting "Om 1, Om 2, Om 3, etc." and help to count the rounds of a cycle for multi-layerd pranayama practices such as nadi shodhana (alternate nostril breath, or nadi cleansing breath)