Expected pileup values at the HL-LHC

In this brief note the expected mean number of interactions and the length of the luminous region for the HL-LHC is discussed for an instantaneous luminosity of $5 \times 10^{34}$~cm$^{-2}$s$^{-1}$. It's dependence on various parameters is discusssed and an uncertainty is estimated. It is found that the mean pileup is about 128, and up to one third of all bunches will have a pileup of at least $140$. The length of the luminous region is anticipated to be about 5~cm.