jack of all trades, master of none
Proverb meaning that having many skills often means one does not excel at any single one.
They say a jack of all trades is a master of none, so it's better to focus on one thing.
A Japanese proverb composed of 多芸 (many arts/skills), は (topic particle), and 無芸 (no art/skill). The exact origin is uncertain, but it reflects a traditional view that breadth of skill can come at the expense of depth.