The HOA doesn't have any set number of trees or shrubs that you should plant in your yard. It only requires that you maintain your yard by mowing, keeping trees and shrubs healthy and trimmed, and keep beds and lawn as free of weeds as possible.
The City of Frisco, on the other hand, does have a set of minimum requirements. Lots in Willow Bay are required to have two shade trees (lots over 8,500sq ft are required to have three shade trees and lots over 10,000sq ft are required to have 4) with a minimum of 3 inch caliper at the time of planting. This means, at 4.5 feet from the ground, the trunk should be 3" in diameter. The trees can be any combination of the following: Caddo Maple, Pecan, Shagbark Hickory, Deodar Cedar, Texas Percimmon, Russian Olive, Honeylocust, Black Walnut, Eastern Black Walnut, Eastern Red Cedar, Red Cedar, Sweetgum, Southern Magnolia, Chinese Pistachio, Texas Pistache, Bur Oak, Chinquapin Oak, Shumard Oak, Texas Red Oak, Live Oak, Western Soapberry, Bald Cypress, Winged Elm, American Elm, Cedar Elm, Chinese Elm, Lacebark Elm, or Siberian Elm. These can be planted anywhere on your yard. Substitutions for these listed trees must be approved by the City of Frisco's Planning & Development Service Landscape Architect.