Not that quickly, Eric Barone worked on the game by himself for over 4.5 years. Version 1.0 was entirely his work. He only brought in other people when the game was ported to other platforms and for the later point updates.
Arthur Lee contributed to the 1.4 and 1.5 releases of Stardew Valley. He's also aired his greviences about being hired as a contractor on the game but then wanting and not getting a profit share for future projects.