  In the simplest cosmological models consistent with General Relativity, the
total volume of the Universe is either finite or infinite, depending on whether
or not the spatial curvature is positive. Current data suggest that the
curvature is very close to flat, implying that one can place a lower limit on
the total volume. In a Universe of finite age, the "particle horizon" defines
the patch of the Universe which is observable to us. Based on today's best-fit
cosmological parameters it is possible to constrain the number of observable
Universe sized patches, N_U. Specifically, using the new WMAP data, we can say
that there are at least 21 patches out there the same volume as ours, at 95%
confidence. Moreover, even if the precision of our cosmological measurements
continues to increase, density perturbations at the particle horizon size limit
us to never knowing that there are more than about 10^5 patches out there.
