If you buy a piece of land what is the percentage that you could build on?

Depends on local zoning requirements, lot lines, set backs, HOA etc. Too vague a question. Very common to find a 2000 sq. ft. rancher on a 1/4-1/2acre lot(11,000 -22,000 sq. ft.)

Many towns publish zoning laws online. But even within one town, there will be different laws for different parts of town.