Other mistake: At the hospital Laurie could not know that Myers was after her in there.
Other mistake: After Myers stabs the blonde nurse the scalpel is clean of blood.
Other mistake: Myers gets shot 6 times in the chest and body but has no blood or bullet holes.
Continuity mistake: At the beginning of the movie, the front door lights on the Doyle house are on, then off, then on again in just a few different shots.
Answer: I have read that, in early drafts of the script, the hospital was a health clinic, not a standard hospital. This would possibly explain why there are only a small number of patients, though it doesn't explain why there is a maternity ward, or why the mother brings her son there for emergency treatment.