(a) Except as provided by Subsection (b), a board is composed of:
(1) five members; plus
(2) the number of additional members determined under Subsection (c), (d), or (e).
(b) The board of an authority created by an alternate municipality is composed of five members.

(c) If less than 50 percent of the population of the principal county, excluding the population of the principal municipality, reside in the authority, the board has two additional members.
(d) If 50 percent or more but less than 75 percent of the population of the principal county, excluding the population of the principal municipality, reside in the authority, the board has four additional members.
(e) If 75 percent or more of the population of the principal county, excluding the population of the principal municipality, reside in the authority, the board has six additional members.
(f) In this section and § 451.502, “principal county” means the county in which not less than 51 percent of the territory of the principal municipality is located.
(g) This section does not apply to the board of an authority described by § 451.5021(a).