Can I use my old sim from a iPhone 6s in a iPhone 12 pro?

Above is correct. Yes, the SIM is the same size.

Your new iPhone must either be carrier unlocked, or the SIM must be associated with the same carrier holding the lock.

Details on carrier locks ➡️ How to unlock your iPhone for use with a different carrier - Apple Support

Potentially, yes ... at least, physically.

Ref: Find out which size SIM card your iPhone or iPad uses - Apple Support

I moved my T-Mobile SIM from my iPhone 6s to my iPhone 12 Pro. It just worked.

Note: I purchased both my 6s and 12 Pro unaffiliated with any carrier directly from Apple. That is to say no carrier lock.