Determine the number of significant digits in the value 1001 .

According to the rules of significant figures, all non-zero digits are significant.

Zeros occurring between two non-zero digits are also considered significant.

In the given value $1001$, the digits $1$ and $1$ are non-zero, and the two zeros are located between them.

Thus, all four digits are significant.

The number of significant digits is $4$.