There are approximately 150 residential, commercial and agricultural properties on the Sandringham Estate, and more than 200 people are employed there. Some of the more well-known major residential houses on the estate include York Cottage (once home to the future George V and Queen Mary and now used as the Estate Office), Park House (childhood home of Diana, Princess of Wales and now used as a retreat for disabled people), and Amner Hall (leased by the Duke and Duchess of Kent from 1972 until 1990, don't know who occupies it presently).
Balmoral employs 50 people full time, and between 50-100 part-time, plus many other self-employed contractors. There are 45 houses provided to estate staff, plus 22 houses occupied by tenants. In addition there are a half-dozen holiday cottages let on a weekly basis (when they are not in use by Royal Household members during the Royal season in the summer).
As far as Highgrove Estate, yes, it is owned by the Duchy of Cornwall, who purchased it on behalf of the Prince of Wales. When he becomes Duke of Cornwall upon his father's succession to the throne, William certainly could occupy the estate, or, if he prefers, lease it out and choose another property. Hard to tell what he might choose.