Here are my own observations.

Villain teams that are relatively small tend to function like well-oiled machines. The group members tend to compliment each other's abilities and use strategic team work.

There is a good chance that there will be bickering, betrayal, and backstabbing among the villains once a team reaches even six members. This is almost guaranteed with seven or more members.

Once the villain team reaches seven or more members, there is likely to be overlap among member's abilities and personalities.

With more members, there is the increased chance that some of the villains will come across as one-dimensional or stock villains.

Once a team reaches 10+ members, the group can come across as more like a disjointed evil army with competing agendas as opposed to to tight-nit and focused team.

Just my two-cents.

I like three to four  as the optimum.  I agree with Baretta; too many and they fall apart, too few and they lose to easily unless they are more powerful than they should be.

With four, you can safely leave someone to be captured while the Chief Baddy gets away only to vex our heroes again when I run out of other ideas.

Looking forward to the Suicide Squad movie?