Xenos worlds are non-human and non-Imperial worlds inhabited by alien races. Most alien races occupy only a single world or a small group of worlds. The majority of aliens are comparatively primitive, peaceful or powerless, and of little interest to humanity. Only a few alien races are powerful, aggressive and possess technology which rivals that of the Imperium. Of these, the most common are the Orks, Eldar, Necrons and Tau.
White Dwarf 139 (UK) (page 9) states that "Humanity is but one of many races in the galaxy. However, none are so widely distributed or so numerous as humans." Contradicting this however, several sources (including Codex: Orks (2nd Edition) page 4) clearly state that the Orks are the most widespread race in the galaxy. A possible compromise of these two opposing statements would be that the Orks are the more numerous race but are split into many fragmented factions, whilst the Imperium is the largest cohesive empire.
The Necrons also believed to be even more widespread than both Orks and Humanity as on (page 17) Codex: Necrons (5th Edition) it says that "These Tomb worlds represent only a handful of the many millions spread throughout the galaxy." and on (page 8) that "even now, at the close of the 41st Millennium, billions of Necrons still slumber in their tombs, silently waiting the clarion call of destiny" which shows that the Necrons could possibly outnumber every other race in terms of planets and numbers.