How many Mythos cards can be played during the investigator's turns?

Question

I am wondering how playing Mythos cards works. I read that the Keeper can play 1 Mythos card in the investigators' turn. Does that mean:

1) when playing against 4 investigators, the Keeper can play 4 Mythos cards (one card each per single investigator's turn) before his turn comes,

or

2) the Keeper can play only 1 Mythos card per a complete turn cycle of all investigators (i.e. the Keeper can play 1 Mythos card before his turn comes)?

Answer 1

The keeper can play one Mythos card per investigator's turn.

From the rules, bottom of the first column on page 16:

The keeper may play a maximum of one Mythos card per investigator turn and may never play Mythos cards during the keeper’s own turn.

Each player/investigator has his own turn each round. Playing against 4 investigators, the keeper could play a maximum of 4 Mythos cards a round.

From pg. 6

Playing the Game

Mansions of Madness is played over a number of game rounds. During each round, each player resolves his turn (starting with the investigators)