In a chemical formula such as C6H12O6 and H2O, what do you call the little numbers below the normal line of text.

subscript number,

superscript numbers

These are called subscript number.

That is the number below the normal line of test are called subscript number.

This number indicate the indicate the number of atoms of the element present in the chemical formula.

In both of these C₆H₁₂O₆ and H₂O, the number written below the line of the text are called subscript numbers.

subscript number