A council’s determined number of National Delegates is based on the number of girls it has registered as of September 30th the year before the regular session of the National Council. Each council gets a minimum of three national delegates: two delegates and then one additional delegate for up to every 3,500 girls after the first 3,500 girls.
USA Girl Scouts Overseas uses the same formula as councils.
However, the 3,500 number can be changed to make sure the total number of National Council members does not exceed 1,500. GSUSA therefore notifies councils of the total number of delegates the council is allowed.
Source: Constitution/ARTICLE IV/THE NATIONAL COUNCIL/Membership.