I know this question is rhetorical, but hypothetically, one should be able to load the same map inside of the child DOOM instance that loads the same map, which would allow you to run as many DOOM instances as you could fit within the CPU bounds of the system running it, or the RAM bounds, up to 2 GB of RAM or whatever the system has, whichever comes first.
It's a shame that (if I remember correctly from the video?) the bug that allows this is restricted to single-player; if not, I suppose it'd be possible to start the "outer" map as a network game, and then use the second instance of the game to join the outer instance over the network (including the possibility to walk around and find the original player character).
Source code: https://github.com/kgsws/doom-in-doom
Gizmodo article: https://gizmodo.com/doom-inside-doom-ii-vanilla-chocolate-ha...