  A variety of observational campaigns seek to test dark-matter models by
measuring dark-matter subhaloes at low masses. Despite their predicted lack of
stars, these subhaloes may be detectable through gravitational lensing or via
their gravitational perturbations on stellar streams. To set measurable
expectations for subhalo populations within LambdaCDM, we examine 11 Milky Way
(MW)-mass haloes from the FIRE-2 baryonic simulations, quantifying the counts
and orbital fluxes for subhaloes with properties relevant to stellar stream
interactions: masses down to 10^6 Msun, distances < 50 kpc of the galactic
center, across z = 0 - 1 (lookback time 0 - 8 Gyr). We provide fits to our
results and their dependence on subhalo mass, distance, and lookback time, for
use in (semi)analytic models. A typical MW-mass halo contains ~16 subhaloes
>10^7 Msun (~1 subhalo >10^8 Msun) within 50 kpc at z = 0. We compare our
results with dark-matter-only versions of the same simulations: because they
lack a central galaxy potential, they overpredict subhalo counts by 2-10x, more
so at smaller distances. Subhalo counts around a given MW-mass galaxy declined
over time, being ~10x higher at z = 1 than at z = 0. Subhaloes have nearly
isotropic orbital velocity distributions at z = 0. Across our simulations, we
also identified 4 analogs of Large Magellanic Cloud satellite passages; these
analogs enhance subhalo counts by 1.4-2.7 times, significantly increasing the
expected subhalo population around the MW today. Our results imply an
interaction rate of ~5 per Gyr for a stream like GD-1, sufficient to make
subhalo-stream interactions a promising method of measuring dark subhaloes.
