The number wasn't deliberately chosen. That's just how many colonies had settled in the new world when independence was declared in 1776. Starting with the first colony, Virginia in 1607, new colonies sprung up (Georgia in 1732 was the 13th) and there were thirteen when the American revolution happened. By the 1790s, three new states had joined the Union. The Constitutional convention was in 1787, when the Constitution was framed.