about the turbo pressure setting

I have a question about the turbocharger system. for stock engine, how many boost is safe, if we upgrade to forged pistons and Rods, how many boost can be set（In case the turbine has sufficient capacity）.

What is the relationship between boost and compression ratio?

Every engine and turbo setup is different so there's no one size fits all rule, what engine are you working on?

BMW N20B20 2.0T ENGINE,

There's a lot more to consider than turning the boost up with that engine, it's direct injection and so can suffer from LSPI. What ECU are you planning on tuning it with?