Don't know if this is the right place to post this but does anybody know where I can possibly download star trek ongoing issue 20
After the issue has been released, you can purchase a digital copy here at comiXology. If, however, you're asking about downloading it without paying for it (i.e., an illegal download), you won't find any answers on this board.
You'll have to wait until it's been released, and at the moment it doesn't have a release date, Comic List has it under the "TBD" list.
In the Ongoing discussion thread in the Lit forum, Allyn Gibson posted that #20 will be out next Wednesday, May 8th.
Which, interestingly enough was to be the original release date for issue 21. But since that is set after STID it has understandably been postponed to the end of May.