antivirus for MacBook Air

No you do not need it and you do not want it. Many posts here about a slow computer can be tied to using antivirus tools.

Therre's a lot there because people (particularly former Windows users) think they need them and if somebody can find somebody to buy something then you can bet they will make it to sell it even if they don't need it.

Please read Effective defenses against malware and ot… - Apple Community.

why there is too many antivirus app in app store ?

The reason is that people are gullible, have too much money, and can be relied upon to throw it at away on worthless garbage they do not need. This is a general fact of life not limited to the App Store.