How to Know sellers activated

How could we know that on particular strike price how many buyer and how many sellers are there???

Market depth is a live feed of best 5 quote, i.e. how many buyers and sellers are present at prices (nearest 5) above and below the LTP or market price.
In app - You can open the stock/scrip details page and there you will be able to see market depth.

in web - on watchlist you will see an icon for market depth beside B|S.

If you are on option chain/portfolio/ordrers - click on 3 dot menu and open market depth.

You can also press F4 key on any selected (highlighted) scrip to open market depth.