how many square feet will 1 gal. cover on new textured siding?

There is not an exact number I can give you. It depends upon the depth of the texture. After the siding has been sealed(primed) the coverage can range from 300 sq. feet per gal. to as low as 150 sq feet per gal. If the texture is coarse or heavy you will likely need multiple coats.

Typically, one gallon will cover approximately 400 square feet. Actual spread rates will vary depending on surface porosity and texture.