How many significant figures are there in the number 100?

There is one significant figure as a significant figure is any number other than 0

if it was 120 then it would be 2

There is only one significant figure in 100. Remember the three rules that make a number significant:
1. Non-zero digits are ALWAYS significant
2. Any zeros between two SIGNIFICANT digits are significant as well.
3. A final zero or trailing zeros in the DECIMAL portion ONLY are significant.

Iwould you figure it out but you don't have a picture or in your question as to what the physical property you're looking for is