A recent paper - Functionality of Varroa-Resistant Honey Bees when used in Migratory Beekeeping for Crop Pollination --
has demography data taken from hundreds of hives.
Observe the chart of brood frame coverage:
data on total bee coverage is similar
At 5-6 frames of brood, you are within the margin of error for commercial hives that are survivors from the year before. Russian bees (RU on the charts) winter over in smaller clusters.
Also note: Bees peaked in number on May 28, and decline as the summer dearth takes hold.
Source:Functionality of Varroa-Resistant Honey Bees (Hymenoptera:
Apidae) when used in Migratory Beekeeping for Crop PollinationAuthor(s): Robert G. Danka, Lilia I. de Guzman, Thomas E. Rinderer, H. Allen
Sylvester, Christine M. Wagener, A. Lelania Bourgeois, Jeffrey W. Harris and José
Source: Journal of Economic Entomology, 105(2):313-321. 2012.
Published By: Entomological Society of America