They might not need 13, but that sounds about right. Consider what their roqdmap consists of:
- Revised versions of every Class from the 2014 PHB [which should take up 3 more packets, through to the end of 2022]
- 36 additional Subclasses [probably another 4 UA, later]
- New and revised Spells [probably spread across the Priest and Mage Class drops]
- New and revised Feats [which Crawford outright says in the videos from this week will be spread across the Class Group UA]
- New weapon options for certain Classes [which Crawford seems to hint is the bread and butter of the Warrior Group in one of the videos]
- New Home Base system for PCs [probably worth a full packet]
- Encounter building rules [definitely a distinct packet]
- New and revised Monsters [probably in the same packet as the new encounter building rules]
So, I'd guess the plan before any revisits is for about 9 more UA packets to cover what they've committed to, bringing them to aboutba year after the start. Then, they can make any revisions suggested by testing for another round, and prepare the final book way before the Holidays in 2024.