With an unknown capacity for the pan try the following.
Once the
oil is drained add 5 qts (starting point and not so much that would be too difficult to remove some of it if the pan capacity is small) and give it about 30 minutes to let everything drain down into the pan. Check the dipstick and see where the
oil level sits. Add
oil if needed until the level is at the full mark on the dipstick. Don't for get to give the oil time to drain down after adding. What you end up with is the capacity for the pan.
Start the engine, check for pressure and check for leaks.
Shut the engine off and give it another 30 min. to drain down. Add oil as needed until the oil level is at the full mark on the dipstick again.
Whatever you have put in at this point is pretty close to the capacity of the entire system.
Personal recommendation: If your filter is mounted vertically fill the new filter with oil before you spin it on.
DonC