It depends upon how you got to the 12, for example:
28.6 - 16.6 = 12.0
Because each of the numbers that was used to get to the 12 had 3 significant figures, you should write the 12 with three significant figures also.
29 - 17 = 12
In this case, each of the numbers that was used to get to 12 had only 2 significant figures, so use only 2 significant figures in the 12.
They all have 1 as the first digit.
The zero after a decimal point considered as significant digit because that is the defined, accepted convention. 12 is 12 +/- 0.5 12.0 is 12 +/- 0.05 12.00 is 12 +/- 0.005 12.000 is 12 +/- 0.0005 etc.
12 has two significant figures. A significant figure is any non-zero digit or any embedded or trailing zero. Leading zeros are not significant.
When the last digit of a significant digit is 5, you always round up.
All digits are significant in this case. Any digit after a decimal point is a significant digit.
The first significant digit is the 5, which is in the ten-thousandth's position.
Four - zeros in-between significant digits are significant.
4 significant digits.
All four digits are significant.
Only three significant digits are correct, because 5.0 contains no digit for hundredths, and such a digit would be required to give four significant digits in the answer. Because 7 + 5 equals exactly 12, a third significant digit is gained for the answer. Note in contrast that the sum of 7.68 and 0.50 would properly contain four significant digits.
Zero is considered a significant digit when it is to the left or the right of the decimal point. It is also a significant digit when it follows a non-zero digit or when it is between two digits.
Four - zeros between significant digits are significant.