Will installing kernel 5.12 from 5.9 break anything

just curious is I should bother upgrading the kernel, if there is any performance increase or just more hardware support

You should at least switch to 5.10 since 5.9 was already removed from the repositories.

I would recommend to install both, 5.10 and 5.12.
Then reboot and remove 5.9


if I install both 5.10 and 5.12, which one will be used? or can I choose after reboot

By default the latest one so 5.12.

You can choose which one from the grub menu (press shift during startup to show grub)
So in case you run into issues with 5.12 you can still boot with 5.10 or in case that gives issues 5.9 as well.

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5.12 mostly provides better hardware support, for example for wireless lan adapters but it is still pretty fresh and could be a bit buggy. if you have no issues with it better stay on 5.10 or 5.11 for the moment. They will be kept up-to-date for a while. As @moson mentioned, 5.9 is frozen and should not be used any longer.