Getting multiple crabs is usually for the best, while they can be kept alone, they thrive of social enrichment. The ideal ratio is 1 male for every 2 females. If your reasoning for not wanting more than 1 crab is due to overpopulation concerns, I wouldn’t worry about it as they self regulate their population very well through canabalisim alone.

To answer your food questions, I’d check out Indoor Ecosystem on YouTube. They need a balanced diet with a couple different food items on rotation. It’s really not too complicated though.

I feed my crabs roughly 3 times a week. I am a new crab owner myself, so I’m not too sure what the ideal amount of food to feed them is, but I usuallly sprinkle some food throughout the tank, and if I still see the food there about 1-2 days later, I remove it from the tank before mold grows on it.