Yeah, we tend to have a "set" number of fibers. There's I think a small percentage of change maybe.. elderly probably.
Most of the slow/fast twitch has to do with the amount of hypertrophy in each due to specific sports training (power/strength vs endurance) and the subsequent other adaptations of the muscles to those stressors (increase glycogen, glycolytic enzymes, etc. vs. increase mitochondria, oxidative enzymes, etc.).
Also.. "Gram for gram, the two types are not different in the amount of force they produce, only their rate of force production." Most people don't know that but it's true. I think it has to do with some different isoforms of myosin being able to contract their heads faster in response to nerve impulses.