Some users want to be able to self-delete. Is it possible to allow this?
They can, until they make 2 posts/topics.
Can this number be changed?
No it is hard coded.
The idea is, if they were active enough to have more than that, deleting their account would potentially render some topics (that they were involved in) useless.
Self service delete for users with lots of posts is not desirable, it can be very disruptive to forum members. They can simply contact a moderator who can take care of this if needed either by anonymizing or deleting.