In my experience, one drum is pretty adequate.

Most fields in the US are limited in length, so the launch height is usually
just enough for a pattern. I find with one drum I can launch a ship, retrieve
the chute, and have be back at the launch point just as the glider lands. We
turn the glider around (usually lands cross wind), and we're ready to go with
another. We achieve about 6-7 launches per hour this way at my field..until
two or three other people turn up (with little experience but they're BS'ed the
membership into thinking they know what they're doing) and our rate goes down
to about 3 an hour.

Jim Vincent
CFIG
N483SZ
illspam