In the Star Trek universe how many Emergency Medical Hologram's can be online at any one time? For example, in Season 4 Episode 14 "Message in a Bottle" the Doctor engages the Prometheus EMH with no apparent problem, but in the same season in Episode 4 "Revulsion" when the Doctor and B'Elanna leave Voyager to meet another AI, the Doctor mentions that he is leaving Tom Paris as a nurse. Couldn't Voyager just switch on another EMH? It is, after all, just a programme.
(I mention Star Trek universe rather than just Voyager since the EMH turned up in the film First Contact.)