This puzzle has 21 of them Crossword Clue

 The answer to the crossword clue "This puzzle has 21 of them" is BLACK SQUARES.

In a crossword puzzle, black squares are the squares that do not contain any letters. They are used to separate the different words in the puzzle and to create a symmetrical grid.

The number of black squares in a crossword puzzle can vary depending on the size of the puzzle and the difficulty level. A typical 15x15 crossword puzzle will have about 21 black squares, while a 21x21 crossword puzzle will have about 35 black squares.

The black squares in a crossword puzzle are not always evenly distributed. In some cases, there may be more black squares in one section of the puzzle than in another. This is done to make the puzzle more challenging and to create a more interesting visual effect.

I hope this helps!