How often should I shut down or restart my MacBook Air M1 (2020)?

Sleep it. Restart the Mac as a troubleshooting step or for cleaning.

Put your Mac to sleep or wake it

When you’re not using your Mac, you can save energy by putting it to sleep. When your Mac is in sleep, it’s turned on but consumes much less power. It takes your Mac less time to wake from sleep than it does to start up after being turned off.

https://support.apple.com/guide/mac-help/put-your-mac-to-sleep-or-wake-it-mh10330/11.0/mac/11.0

Some Mac notebook computers have a feature called safe sleep. With safe sleep, each time the Mac goes to sleep, the current state of the Mac is saved, including open apps and open documents. Safe sleep prevents your information from being lost if the Mac is completely shut down during sleep, which could occur if the battery runs down or is replaced.

https://support.apple.com/guide/mac-help/what-is-safe-sleep-mh10328/mac

Energy saving

https://support.apple.com/guide/mac-help/save-energy-mh35848/11.0/mac/11.0

First, we are NOT an official source. This is not Apple support. We are all other end users like you. For an official answer, you need to contact Apple.

And, different users have different opinions.

I shut mine down at least 2 - 3 times a week or more as it goes through some maintenance and checkups when booting up. Advertising energy savings is somewhat irrelevant compared to the fresh start it gets when booting up. Personally, I prefer that.

Others do not.

I do not believe there is an "official" must/should follow rule. It's your preference. But, if it exhibits some wonky behavior, do shut it down when done for the day.