i just wanted to know how many significant figures in the number 100
One is the correct answer.
However those zeros to the left of the decimal point are a problem in defining "significant figures". The person who wrote it may have gotten 101 for another experiment to the same accuracy and then you would have said, "Oh, that is three significant figures."
The safe thing is to write
1. * 10^2 if you mean 100 +/-50
and write
1.00 * 10^2 if you mean 100 +/- .5