I think he meant the number of uses left from one dose of poison.
There is no dynamic data for this information, so it can not be added to the maintained window. I would like to have this data also with other spells like reactive healing and some of my Conji spells.
We should all feedback that SOE adds this information (with 0 or empty for spells without charges). If enough people send a feedback SOE might consider to include this information into the dynamic data.