Up to Orthodontist. The only person to accurately answer this question for you is your orthodontist, as each situation you described is unique to the individual.
The least. After careful diagnostic records are reviewed and a best plan for that individuals treatment is established: the only removal criteria to be considered is the least amount of teeth to be able to create a beautiful alignment, in the context of facial proportions for the patient and in such a way as to allow for the most effective and healthful occlusion (bite) of their teeth.
Need full exam first. A full exam with proper x-rays, photos and models is needed before anyone can say for sure. It will also depend on the current health of all your teeth and gums. For instance, it may be better to remove a tooth weakened by bone loss now, get the others properly aligned, and then replace whatever is necessary after braces.
Depends. 1 or more. Depends on where the crowding is located, the amount of crowding, and all the other diagnostic factors. Sometimes crowding can be resolved without extraction. See a qualified orthodontic specialist for evaluation and accurate answer that pertains to you.
Consult. See an orthodontist for a consult. Sometimes straightening teeth with braces requires no teeth to be pulled. Other times it does. It depends on what you and your orthodontist decide is the best treatment for you.