You may receive up to 3 instances of Mercenary Operations per day, with a delay no less then 6 hours between each refresh.
In other words, if you complete all your Mercenary Operations of a certain type, you will receive your next portion of Mercenary Operations in 6 hours.
However if you have already completed 3 instances of Mercenary Operations during the day, you will have to wait till beginning of the next day, to receive new Operations.