After denied i 485, can i apply again? how many times? If fail, what effect to ?

One can always reapply or file a motion to reopen. However, the more important question is on what basis did you apply? Was it through family, employment, asylum, etc.? Why it was denied is also a big issue. Was it because you simply missed a fingerprint appointment? Or was it because of criminal or other issues which may not be easy to fix. You definitely need to talk with an immigration attorney immediately as there are time limits to filing a motion to reopen.

I agree with Attorney Amos, we need more facts, the reason for the denial is very important as well as grounds for adjustment. You must see an immigration attorney to review your case.

There are many reasons that adjustment applications are denied. You should not just file again until you have corrected the reason that you were denied. Understand that USCIS keeps all of you paperwork in one main file and sometimes temporary files to be consolidated in the main file. So if you were denied - it does not meant that the next time around the next officer is not gong to know all that happened. As another attorney noted - sometimes you can file a Motion to Reopen if you think that the first decision was in error, but in reality, an attorney should review all of your documents and advise you after understanding everything in your case.

The denial of an I-485 is without prejudice to filing a new application. Whether that makes sense, of course, depends upon what it was denied in the first place. Consult with an experienced immigration attorney who can review your case and advise you how best to proceed.