A developer has proposed building 48 townhouses on 12 acres at Mainland and Wambold Roads in Towamencin.   W-B Homes shared its vision for the site with the township supervisors Wednesday night.   Each unit would measure about 2,000 square feet and have a basement as well as a garage.   Company officials expect them to sell for less than $300,000.   The township supervisors would need to rezone the proposed construction site for multi-family residential use before the project could proceed.   The land is currently designated light industrial.