Basically with Reversed Incremental mode the retention policy is a straight forward thing. If you define 7 restore points to keep, then you'll have 1 VBK and 6 VRB files.
However, with Forward Incremental backup mode it is getting a little bit more complex. Generally speaking you'll have twice more restore points available on the disk compared to the retention policy settings.
Tom has posted a pretty detailed explanation on the previous page of this very topic. In addition to this, please look through these topics more carefully: v5 Restore Point Bug?
and Job Set for 14 Mount Points - 46 exist
On top of that, what is your retention policy now (still 21?) and how have you scheduled your backup jobs?