Answers.

Is 回美证 application will be affected by 排期?
No!

I submited I485 end of last year but didn't apply 回美证, now, I think I have to apply it because who know how long will take to get immigration number. Can I apply 回美证 seperate I485?
Yes!

How long will take to get approved?
Usually several weeks to 1-2 months. But it can also be shorter or longer.

How many times I can you it in a year?
If you apply multiple entry, which is usually the case, you can use many times.

How many times I can apply it before I get green card? thanks!
One is usually valid for one year. You can apply as many times as possible.