S1-AP is only TL association between eNodeB and MME.
According to 36.413, S1-Setup-Req only contains Global eNodeB id, Tracking Area codes and PLMN ids.
I am not very sure if an S1 link would carry SIB messages as it is something to do between eNodeB and UE as far as my knowledge goes and it would not have any bearing with S1-AP as such
Even if SIB info would be carried over S1, it would go as one of the IEs in the S1-AP message, whereas the S1-AP setup would always only mean setting up the link between eNodeB and MME and it would not have any bearing on the number of cells it is serving. Corrections welcome.