New Hampshire: 91% enrollment retention rate through May
Sometimes I don't have anything particularly useful to add to a data point. New Hampshire is one of the very few states which don't operate their own ACA exchange which does keep track of (and, more importantly, report) ACA exchange enrollment on a regular basis, via a monthly report.
New Hampshire enrolled 44,581 people in individual market QHPs during open enrollment this year, so the 40,728 enrolled as of May shows an impressive 91% retention rate.
Their SHOP enrollment is around ~1,300 people working for ~230 small businesses.