Every time a new gate opens on the board, it also spawns a monster. Every time a gate opens where a gate is ALREADY open, you get a monster surge, which I believe spawns either as many monsters as there are open gates or players, whichever is greater. For example, if you were playing in a group of 5, and you had 3 gates open, and you already had a gate in the Graveyard or whatever. If you drew a mythos card that indicated you open a gate at the graveyard, you would instead draw 5 monsters from the cup and place them as evenly as possible between the open gates. You also need to take into account how many monsters can be in Arkham at one time, and move any "overflow" monsters into the outskirts.
Thanks so much!
I was kinda slow waiting for "a monster appears" on mythos cards. In the end we had only two lonely monsters hanging out on the board.