  Ultralight bosons can form large clouds around stellar-mass black holes via
the superradiance instability. Through processes such as annihilation, these
bosons can source continuous gravitational wave signals with frequencies within
the range of LIGO and Virgo. If boson annihilation occurs, then the Galactic
black hole population will give rise to many gravitational signals; we refer to
this as the ensemble signal. We characterize the ensemble signal as observed by
the gravitational-wave detectors; this is important because the ensemble signal
carries the primary signature that a continuous wave signal has a boson
annihilation origin. We explore how a broad set of black hole population
parameters affects the resulting spin-0 boson annihilation signal and consider
its detectability by recent searches for continuous gravitational waves. A
population of $10^8$ black holes with masses up to $30\mathrm{M}_\odot$ and a
flat dimensionless initial spin distribution between zero and unity produces up
to a thousand signals loud enough to be in principle detected by these
searches. For a more moderately spinning population the number of signals drops
by about an order of magnitude, still yielding up to a hundred detectable
signals for some boson masses. A non-detection of annihilation signals at
frequencies between 100 and 1200 Hz disfavors the existence of scalar bosons
with rest energies between $2\times10^{-13}$ and $2.5\times10^{-12}$ eV.
Finally we show that, depending on the black hole population parameters, care
must be taken in assuming that the continuous wave upper limits from searches
for isolated signals are still valid for signals that are part of a dense
ensemble: Between 200 and 300 Hz, we urge caution when interpreting a null
result for bosons between 4 and $6\times10^{-13}$ eV.
