How does the Stun mechanic work in this scenario in Maximum Apocalypse?
The second monster will not be stunned.

The fire extinguisher is an Instant Action, so you'll need to discard it upon use. When you discard it, you take the action that's printed on the card. In this case you stun all monsters on the same space as your character as per short range rules in the rulebook (BGG account may be needed): 


  Short Range: These cards can target and affect the space that your character currently occupies.


You then move to a new tile, where the fire extinguisher was not used, and draw a monster. This monster (a) wasn't in range when you used the extinguisher and (b) you'd already used the effect in a previous action and discarded the card. The second monster will not be stunned.