To trust a member based on number of posts, you will need to configure the forum's permissions and set the number of posts required before a member is trusted as shown below.

If you select "Trusted Forum Members Only" for who may post, then only trusted members will be allowed to post.
If you also check the box "Require approval..." then non-trusted members will be allowed to post, but those posts will not be shown to anyone except the forum owner and forum moderators. These posts must first be approved by either the forum owner or moderator before they will be shown to any other forum members.
If, after a certain number of approved posts, you would like to automatically trust the poster, then you can select the number of posts in the "approved posts" box. After the member has posted that number of posts and had those posts approved, he/she will be considered trusted and will no longer require his/her posts to be pre-approved.
If you set the number of posts to zero, then members will never be trusted based on posts. The only way a member can be trusted is if you explicitly trust them as explained in the previous post above.
The "Require Approval..." option has no effect on "New Topics." If set for "Trusted Only," non-trusted members are never allowed to start new topics.