I'm 21 weeks pg and cervix shortened to from 3.5 to 1.5cm in 3 wks. I'm on hospital bedrest no contractions. How many weeks is a realistic goal for me?

Term would be best. Do as your OB says, consider weekly 17-oh Progesterone injections (or vaginal suppositories) to help prolong your pregnancy. Also receive a course of beta-methasone @ 24 weeks to help with prematurity problems & do what you can to keep pressure off your cervix. 24 weeks is rhe first week of viability but risky; remember each additional week helps. Over 35 weeks would be awesome. Good luck.
Depends. Is this your first pregnancy. You have what amounts to a dynamic cervix. There is differing opinions on how to manage this. I would follow it closely with sonograms, assessing if it continues to shorten, funnel, change with fundal pressure, etc. I would talk with you about the pros/cons about 17oh Progesterone injections in this case. Also consider prophylactic antenatal steroids.