A 9 × 9 Sudoku needs at least 17 given digits to have exactly one solution; with 16 or fewer it always has more than one.
McGuire, Tugemann and Civario searched every case by computer and showed in 2012 that no 16-clue puzzle has a single solution. Puzzles with 17 clues do exist, so 17 is the floor.
17 is the least that can work, not a number to aim for, and the clue count says little about difficulty: on this site Hard and Expert puzzles leave about the same number of clues, and what differs is which clues are left.
Estimate with the rule, then check it against the calculator that models it properly.
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