Most super-villains feel they can take on the world. They feel the world is for them to rule. A lot of villains feel a sense of superiority and tend to avoid working with other villains on teams. Because some tasks in a villain's plan for world domination are beneath them, it falls to their henchmen to do the small things to make the plan happen.
The problem with having henchmen is, you can't simply put out an ad in the paper. Villains are usually wanted by the authority for past crimes. They can't just list their home phone number for the world to see. They can't just hire any random person either to be one of their thugs.
Let's think about the qualifications first. A supervillain should look for someone smart, but not too smart. The last thing they need is someone to try to out-villain them. At the same time, we usually see villains using thugs that are barely able to comprehend how to tie their shoes. That's when plans usually fall apart. The right henchperson has to be smart enough to understand the basic elements to their evil scheme and have the ability to carry out any technical tasks the villain needs them to do.
Fighting skills and/or strength are necessary. A true supervillain needs their henchmen to be able to hold off the hero while the final stages of their dark plan take place. A good henchman can also double as protection if the villain gets outnumbered or outgunned. Of course the villain wouldn't want the hired help to be too strong. There would be the risk of them refusing to take orders or carry out their part of the plan.
To make a lot of the above happen, a villain can either look for hired help that is easily intimidated (threats of death to them or their families might work) or they can throw a lot of money at them. Money isn't always an option up front as the villain probably just broke out of jail or is in the process of remodeling the new evil base of operations. If someone is willing to work for a vile and evil person, what does that say about their sense of honesty?
A well prepared supervillain considers all these qualities but there's still the question of where can they locate the henchmen? Organizations like AIM, Hydra and the Hand seem to have countless numbers of disposable...hired help. Do they offer really good health benefits? What makes them so loyal and where do they all come from? We know that Taskmaster did train villains in the past. Do these henchpeople strive to be underlings for the big villains? Are they hoping this is a stepping stone in their evil career? Maybe it's because girls always like the bad boys...