I'm trying to connect a disk from a brake to a shoulder using self locking screws but cant find a formula that would help me to calculate the torque these screws would be subjected to or how many bolts I would need. What do I need to do?
You have to first calculate the force you'll need at the interface to prevent slipping. This will be dependent on your external loads (forces and moments).
Using this force and static friction between the two parts, you can can calculate how much normal force will be required to prevent the joint from slipping.
Based on the normal force, choose a set of bolts that can take it. Design the bolt pattern such that you're only utilizing about 60% of their ultimate strength. For example if the required force is 10kN, you could choose 4 bolts rated at about 4.5kN or 5 bolts rated for 5.3kN. It will depend on your geometry and whatever other requirements you have.
Once you've chosen the bolts, you can calculate the torque spec to meet about 60% of their rated strength.
Is this a disk brake in an automotive application?