LrC 12 doesn´t start. it crashes all the time om my M1 Mac

The first thing that you should try is to switch off the GPU support from the Lightroom preferences and check if that helps to fix the issue.

Go to Lightroom > Preferences > Performance tab > Uncheck "Use Graphics Processor" > Restart Lightroom.

https://helpx.adobe.com/lightroom-classic/kb/lightroom-gpu-faq.html#troubleshooting

https://helpx.adobe.com/lightroom-classic/kb/troubleshoot-gpu.html

If Lightroom doesn't start correctly the please see "Solution 2" in the document behind the second link.

Troubleshoot GPU issues | Lightroom Classic (adobe.com)

Another step is to try to reset the Lightroom preferences.

https://www.lightroomqueen.com/how-do-i-reset-lightrooms-preferences/

It's recommended to backup your preferences before you reset the preferences to the default settings:

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html

Follow the instructions at the link below.

https://community.adobe.com/t5/lightroom-classic-discussions/quick-tips-how-to-give-full-disk-access-to-lightroom-classic-on-macos/td-p/12913413

I addition provide some more information about your system.