I once tried to catalogue the marines in the books. I found the Master of Vigilance, 5 other Watch-Captains, 1 Chaplain, 3 Apothecaries (1 is also one of the Watch-Captains and 1 is a lone Kill Marine), an Epistolary (it is, however, stated that most Deathwatch Librarians are Lexicaniums), and 2 Forge-Masters. One of the Watch Stations has 40 Battle Brothers assigned to it but most have a single kill team. Some Watch stations have a single Battle Brother or none at all, being entirely automated. There are supposed to be many more Watch Stations than are illustrated on the map.
Off hand, I recollect reading that a few score marines are at Erioch at any given time, though it has the facilities to house an entire chapter.
While kill teams might be customized for a mission, efficient deployment demands that multiple kill teams deploy from a single voidship and kill teams are chosen from those 12-20 battle brothers.
Given these numbers it would appear that Erioch is home base to at least 200 marines and quite possibly 1000. And these are still stretched ridiculously thin covering the hundreds of thousands of systems that are located in the Jericho Reach. Keep in mind that the galactic disk here is approximately 1000 light years thick, and that wouldn't include all the halo stars. It's easy to forget that 3rd dimension on the maps.
It doesn't seem unreasonable that the Ordo Xenos has requisitioned an entire chapter worth of marines. The Ordo Malleus has a chapter dedicated to its service. Of course, the Ordo Xenos has a lot of other places to cover besides just the Jericho Reach. This alone leads me to believe the 200 figure the most accurate, and that's assuming the Jericho Reach is of utmost importance in the galactic scheme of things. Given that the Deathwatch maintained a presence here for 5000 years while the Imperium wasn't present, it seems that's also not an unreasonable assumption.