2300.m has four significant figures.
Two of them
It has 2 significant figures. However, the fact that it is given as 2300 mm and not as 230 cm suggests that it is, in fact significant to 4 figures.
Two significant figures (2,3).
4 significant figures.
Five significant figures.
two
5 of them.
Three
1
Rice crispy squares? A lot! --- but --- 2300 sq ft would be 2300 squares, each of which is 1 ft by 1 ft; or one square which is sqRt( 2300 ) ft by sqRt( 2300 ) ft. - where sqRt( 2300 ) is the square root of 2300, which is, to five significant figures, 47.958.
Six of them.