I guess this is for Number only (adding power consumption and such!?).
It might be possible that it worked on OH1, but not on OH2 anymore!?
Because in OH1 all the switches were handled like (0/1) and could be stored in rrd4j (and displayed in graphs based on that).
In OH2 switches are just handled like ON / OFF - so this might be the root cause (just a guess)!?
I just experimented with Group:Number:SUM in OH 1.8.3, with a group of Switch items. (edit - in fact it was group of groups, the subgroups contain the Switches and are themselves switch-type groups)
This does indeed count the number of Switches that are ON.
Assuming that is intended behaviour (and it does seem a reasonable thing to do) then if that doesn’t work under OH2 it is a regression failure. You should create an issue in Github.