How often should I shut down my MacBook

There is no need nor any benefit to routinely shut down any Mac — or even to restart it for that matter.

None of mine are ever intentionally shut down. If you anticipate not using a portable Mac for a very long time (weeks) then sure, shut it down.

It depends. Sometimes you can go a long time without needing to reboot, but other times you may find a reboot is necessary to restore some performance or resolve some issue where something has gotten into a weird state. Over the years I have gone from never needing to reboot to where I need to do so about every week or two. If you are not having any issues, then you should be Ok without needing to reboot. Sometimes just quitting a particular app & relaunching an app is all that is necessary to fix some issues.....especially with web browsers.

If you are going to have the laptop running on battery, then a full power down may be a good idea so that you are not surprised by an empty battery when you go to use the laptop next time. The longer you leave the laptop sleeping on battery power, the more likely the battery will be low when you go to use it next time. Macs boot so quickly that it really should not matter.