2 of them.
37.753 rounded to one significant figure becomes 40
40
Rounded to one significant figure it becomes 40
40
39 remains 39 when rounded to one significant figure.
4
40 is no more significant than other numbers, and considerably less so than numbers such as π or e.
To one significant digit, 37.19 is 4x101. Writing it as 40 makes it unclear as to whether you have 1 significant digit or 2.
6276 as a significant figure would be 4 significant figures.
1000 is written with one significant figure, with only the 1 being a significant figure.
The significant figure 2.00 has to do with the certainty of a measurement.
It has 1 significant figure.