Each team has 5 active players and up to 1-2 substitutes (varies by tournament); Common roles: In-Game Leader (IGL), AWPer, Entry Fragger, Support, Lurker; One coach per team; coaches may only communicate during timeouts and halftime (Valve coach bug rule)
Why people argue about this
People often think that substitutes can replace players in any order they choose during a match, which leads to misunderstandings about their role and how they're used. In reality, substitutes are only allowed to enter when a player is injured or ejected, and their entry slots are limited by the tournament rules (up to 1-2 per team), not chosen at will by coaches.
