So I've been watching speedruns for a while so I finally made an account on here. I'm going to be speed running a game on old 3DS XL and plan on recording by mounting my phone (as opposed to buying a capture card). So my question is 1.) Do I NEED to have a clock? like is it required? 2.) can I add the clock after my run? 3.) Is there anything else I should be worried about or that I should keep in mind?

1. Each game has its own rules, but generally no, they're primarily for yourself and any viewers.
2. Generally yes. It's always a good idea to consult your game's moderators.
3. Don't forget to read the Game and Category Rules for your game.