I tried to search on the best of my capacity and I am confused.

I have the intel version 11th gen Intel i7-11800H

What is the maximum capacity of SSD per slot? currently the installed SSD is 1TB(with windows OS), I want to upgrade to my second SSD as the laptop supports 2 SSDs in total, I'm planning to buy the 2tb but others say it is up to 1tb only so I would like to know before I regret buying the wrong part and not compatible to my system.

Thanks in advance.

Crucial tests and guarantees what they list for a given machine model.  They show up to 4TB drives for you laptop.

https://www.crucial.com/compatible-upgrade-for/lenovo/lenovo-legion-5-pro-16ith6h

You don't need to buy from them, but you can usually trust their specs.  Whatever vendor you choose, the drive will need to be single-sided.

Zoltan