How many headset spacers might I need?
You can get a close approximation if you have access to a trainer. Install the bike in the trainer with the fork and headset installed. Using a front wheel riser block will make the fit closer to your actual riding position. You can now install the stem at any height you like. When you find something close measure the gap between the bottom of the stem and the top of the headset. I would get a variety set. Most come with a 2mm,5mm,10mm and 20mm. I just did this myself and I got an extra 20mm that I installed above the stem.This allows for some flexibility later. The spacers can be shifted for a different fit for different riders or resale. It is easier to cut the steerer later than try to make it longer.