This has nothing to do with the WN9 calculation itself. It is possible for any rating to be calculated for a subset of battles, it just depends on what parameters to be put into the formula.
Wargaming has no API that returns stats for latest n games, they only return players total stats per tank per battle mode. It is possible to create a solution that retrieves players stats on a regular basis, like every month, and then calculate the rating for that period by subtracting previous stats to current stats. It will be hard to create a solution that calculate the rating for a fixed number of battles though.
The software Wot Numbers has this possibility though, as it reads every battle into a local database.