。
Too many cooks spoil the broth.
Literal
Cook [subject-が] too-many-と soup [subject-が] fails.
A direct Japanese rendering of the English proverb 'too many cooks spoil the broth.' 多すぎる ('too many') uses the ~すぎる excess suffix. The conditional と ('if/when, whenever') here gives the 'when X, naturally Y' proverb-reading. できそこなう ('turn out badly, fail to be made well') is a compound verb: できる + そこなう ('fail, miss').