Those are just the positions in the memory of the web interface.
The web interface completely removes planes and its positions if they haven’t been seen for a couple of minutes.
That history count is a leftover debug tool, it correlates with the webinterface performance somewhat.
For getting UAT planes into VRS see this post (or the entire thread)
UAT Translation from Dump978-fa - #11 by KC1IUC
Anyway if you want some more details on when you saw planes on UAT, use my graphs:
GitHub - wiedehopf/graphs1090: Graphs for readsb / dump1090-fa / dump1090 (based on dump1090-tools by mutability)
UAT graphs are at the very bottom.