This time i want to get quest season unending how to get it, how many holds should i get , should i join either stormcloak or imperial or refrain from joining any faction?

It's part of the main quest, and I'd do it before having anything to do with the CW, but you can still do it if you start the CW, just not if any side has already won a lot of the holds.

Edit: If you are particularly interested in this quest you could make a save before starting it and then you can revert to it to see all of the different outcomes that are possible (along with a bunch of different dialogue).

You can do it even if you've started the CW quest line. I just did it after having taken Whiterun and Falkreath for the Stormcloaks. The scenes are different so it's worth seeing it at various stages.

As for how to start it, just do the main quest and at the point where you go to talk to the jarl of Whiterun they will say they can't help you with their city still under threat.