It’s unusual for a player to cast a spell in response to their own spell, and that will be specifically called out when it happens. Under normal circumstances, if a player casts a spell, it is assumed that the player passes priority afterward. If that player casts another spell, it is assumed that each other player has also passed priority, which caused the first spell to resolve and the first player to receive priority again. Unless the first player says otherwise, only one spell will be on the stack at a time in that scenario.