how many times does a fish opens and closes its mouth in a minute?

Depends how active they are and the size. or worse case it is the water quality, if ammonia and nitrite levels get high it can effect the fish and make it look like that are gasping for air. best way to fix this is with a 25 % water change and to clean the filter.
 
that question is the equivivent of asking how many clouds are there in the sky... it really depends on the fish, temperatues, water conditions ETC. like the other people say, if your fish is breating rapidly, try doing a water change. you likely have nitrites since it doesnt seem you know a whole lot about fishtanks. also, if you have Prime, try adding a bit of that in there.
 
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