Bob, counts for the number of bees on a frame (or seam) run from around
1800 to 2400 (surprisingly, lower in cold weather in my best data set, and
highest for packed brood frames during spring).  So 18,000 works out to
about 10 seams.

This is also the cluster size that my Beeologics data support as the
"target" size of colonies over winter in Calif, when one compares October
strength to December strength.  No matter the October strength, all
colonies tended to go toward about 9-10 frames of bees over winter
(provided good husbandry).  Larger October colonies lost population down to
~10 frames; smaller colonies lost very little, and often grew (with normal
husbandry).

This cluster size is also the same as that found by Seeley as the preferred
cavity size for swarms, and is roughly equivalent to a Langstroth deep full
of bees.

In your cold climate, one would certainly expect that there would need to
be a minimum cluster size to maintain thermoregulation.  Your 5 seams may
be that minimum.  Are you wintering outdoors or in a temp controlled shed?

-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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