how can i top my plants to make the stem split in two?

at what age should i top my plant to create multiple stems?
i want to keep cutting it and make the plant really bushy

I would say at the second or third set of true leaves..

If you have enough time and space, wait until you have 5 sets of true leaves and then use the FIM technique with a super sharp blade and you will get 2-5/6 tops instead of just two liek you would from regular topping. PLus if you don't do it correctly you will either get two branches lieka regular top, or it will continue to grow normaly if you didnt cut enough in which case you jsut have to cut it more. Should be within a week that you will see how many tops you'll get from a FIM.

You can do it over and over to the branches to get a ton of tops depending only on how much space you have and how much you can support the weight of the plant unless it has a massive thick main stem.