You won't know what your post-surgical flexion will be until, well, post-surgery. I too had a very bad knee (my left) that bent very little. Post-surgery its flexion was better than before the operation. I was surprised, but hey... good surprise! If you can, try not to fret about things ahead of any need to fret about them.
Before surgery, I fretted about pain (turned out not to be a problem given the pain killers). I fretted about stairs (turned out to be no problem at all). I fretted about lots of things, and most of them never came close to being a problem.
Physical therapy in the very beginning need only consist of gentle bending (heel slides), ankle pumps (to help prevent blood clots), and getting up to walk to the bathroom or for getting a snack or water. Also you will learn to go up and down stairs, though you won't want to do this more than a couple times a day at first, and get into and out of a car. These are things you need to know how to do, and how best to do them. Otherwise, your knees will need nothing except time and the opportunity to heal.
That will be your job: to give your knees the best possible situation in which to heal. I can only speak from personal experience, but I was conscientious about keeping my knees mobile. I bent them in the recovery room, just to see how well they worked. I did gentle heel slides. Lots of ankle pumps. I walked. The knees do need to move; it's what knees are for, what the joint is built to do, whether original bone and tissue, or with an implant. So I moved them. But I didn't force them.
Knees are pretty good at telling us what works and what doesn't.
Pre-surgery your OS might give you exercises to do. Mine had me do leg lifts (strengthen the quad muscle), pressing the back of my knee into the bed or floor (to stretch my hamstrings for extension), and heel slides. I only learned after surgery, though, that I could have used some strengthening of my upper body too! Would have helped me get up from chairs and toilets.
