So I have 2 sundown uv2 12s the rated power is 1750 RMS. People are recommending putting a 5k amp to the pair and a couple recommended putting 8k amp to the pair. I'm just confused how to know what amp to get
Any decent amp rated at 3k-6k rms at the ohm load you are wiring will do just fine. Keep in mind guys will recommend amps much larger due to rise. A 3k at 1 ohm in the real world won’t do 3k. Amps like the taramps smart series will do close to if not rated power as they make the same power at 1-2 ohms. Whatever you choose be sure your electrical system is up to the task.
You stated you have an SIA 3500 right now. I would run it for now with the supply for the demand, until you decide you want more power and have the supply for the demand for it.
I always say.."Run what you got" You can always make upgrades as time and funds are available/budgeted for. Patience usually pays off in the long run