So... did some more checking. RotC widget says 11/10 settlers/popspace. I counted. There were 9 settlers and 2 brahmin. (just like abernathy who is at 8 pop, the three main abernathies, one regular settler, 1 cat and 2 brahmin). Now of course I am not totally clear on the following:
1) Does leader (MacCready) count as settler? Tried removing him and it did not cause alteration to that number even after travel away -> sleep for 24 hours -> travel back. Now I noticed one of the houses were built assigned to MacCready, and he was not assigned to the matress inside where the city manager desk is.
2) Does provisioner count? That could of course count for one of the 2 missing settlers, as he was away.
3) 11/10 does not appear to be enough settlers to build the last structure in the build plan, which is currently an empty platform with a idle-space table on it. I assume the brahmin do not build houses
4) Are houses built even though not enough settlers are there, or are they built automatically. Lets say 4 settlers in a city with a 20-settler build plan? Are all buildings built or do they wait until 20 settlers have arrived? Different for houses and factories/shops? Because all houses are built, though 1 stand as "for rent", even though pop is stated to be 11/10 (which once again the 2 brahmin would account for)
Most forum posts I have seen outside sim settlements seem to agree on that brahmin, cats and dogs count as settlers. Not sure if Sim Settlements uses the same settler counter as vanilla game though, but they are the same for all my settlements.
Theoretically, assuming that I don't have 2 settlers who have hidden underground, and who do not want to occopy the last house, i figure the brahmin count. In that case, filling settlements that countain animals would require manual intervention to fill for the last spot if you have "build plan limits rectruitment" option turned on.