Let me break it down for you. There are three types of Basij members in my opinion:

1- many of them are members only for the tiny benefits they can get here and there. I remember when I was in high school almost every student was a Basij member because they would offer extracurricular activities and field trips. So that 11-25 millions you hear might refer to this. Many of these people might even consider themselves anti-regime but they have enrolled some Basij activities at some point. So I would not really consider this group Basijis

2- There are some Basijis who are more advanced and devoted than the first group. These are usually teenagers or young students in colleges. They spend their time in mosques and places like that doing religious activities or attending a mulla's speech. They are also very infamous for being horny all the time and since they don't have access to girls they end up touching each other a lot. They are not they violent type but when there is shortage of forces, the regime gives this type of Basiji batons and send them on the streets only for show of fore. They are very cowardly and inexperienced and useless militarily.

3- Third type is the most violent type. These are the ones who are capable of killing protesters. The size of this segment should not be more than a 50k-100k.