counting certain bars on chart

My response is limited because your question contained virtually no details.

plot numberOfBars = BarNumber();

This will plot the count of ALL bars on the chart. From left to right, starting at zero.

If you want to only count “certain bars”, well we need to know a bit more detail. Did you take the time to use the search before posting your question? If you had searched the word “counter”, you would have come across this previous post: https://www.hahn-tech.com/ans/how-does-one-use-a-counter-variable-in-thinkscript/