It looks way overstocked(typical), but if your filtration was up to par and you were religeious with your water changes.... I would guess no more than 8-10. That is a high number based on good maintenance of the tank. Just start with a few and go from there.
That has a wide range of answers from different people. There's an old saw about they have to be crowded but that turns out to be pretty dumb! If you've got a group of bullies does it make them behave if you overcrowd them or do they act better if they all have space?
You have to know which cichlid and what size before saying how many. All male does cut some of the discussion. I've had as many as 90 in a 75 with no trouble and as few as 6 and lots of trouble. Mbunas with lots of rocks and lots of hiding, I could go as high as twenty depending on type. Open water calm types like Protomelas might only be good with ten.
A crowded planted 20 with Hap ahli growouts and adult Lab. chisumulae who all did well as evidenced by the holding female at the top.
An attempt at a planted 125 that did not work out. There were about ten Protomelas insignus at about 8" with a mixed bag of smaller fish. It did not work for me as the insignus spent all their time finding who was boss. I don't tolerate splashing water in the floor and all that noise.
I still have the tank but cut back to only two male insignus and other fish. One large, one small and everybody knows who is boss.
No firm answer for how many will work in a 75. I would go for twenty and adjust as needed.
I'd suggest 12 to 15 to start. While the Peacocks of Lake Malawi are generally peaceful, some, such as Aulonocara jacobfreibergi can be pretty boisterous. Some haps would also be nice in a male only tank. Good luck with it.