1. It is not good for the painting.

2. It is not good for the paintings.

(Are both OK? Which one is frequently used when we mention many pictures in a museum.)

3. How was the first day of school?
It was good. So many of us changed.

(In the dialogue, what is the meaning of 'So'?)

2 answers

Both 1 and 2 are correct. #2 is used when referring to more than one painting.

In 3, the word "So" is modifying "many" -- telling to what degree. It's simply used for emphasis: Not just a few of us changed; MANY of us changed!
2. is correct, paintings indicate more than one.

3. so in that context means to a great degree, so many then means great many. The same meaning would have been conveyed by "A great many of us changed."