Do you want the sensible answer or the "GW didn't really think it through when they did the 6thE Inquisition codex" answer?
When the 'Clave last had this discussion (which I can't find, so it may even have been on the last version of the forum) the reckoning was that on average there was less than one Inquisitor per world, but more than one for every ten. (Between 100,000 and 1,000,000 Inquisitors, basically).
Fewer than that and it starts to get really contrived when they keep running into each other. More than that and you start wondering how they ever manage to get along.
We reckoned that about half of those would be "field" Inquisitors (taking direct action in person) and the other half would be "desk" or "political" Inquisitors (working through others) - with perhaps a few retired Inquisitors, as much as an Inquisitor ever can be/would be retired.
Per sector? I'd say sectors tend to be closer to a hundred worlds than ten (you'll see a lot of Fermi estimation here). Ergo, there's probably between ten and a hundred Inquisitors in an average sector.
Per Ordo, the largest Ordo is Hereticus, and I'd guess is probably about 40-50% of the total.
Xenos is probably some 20-30% of Inquisitors.
Malleus is the smallest of the Ordo Majoris (given that even amongst the Inquisition knowing too much about daemons and the warp is a mite taboo), probably 10-15%.
Of the Ordo Minoris, most are probably in the region of a percent or two of the total.
As far as support networks? Probably thousands (even ignoring things like the crew of any ships they own). Most Inquisitors will have multiple "high ranking" agent cells, then lower ranking cells, reserve agents, agents who don't even know they're working for the Inquisition, informers, information brokers, overseers (to co-ordinate it all) and more.
The "GW didn't think it through in the 6thE codex" answer is contradicting old background (some total BS about the Ordo Hereticus having existed as a secret, even within the Inquisition) and tiny Ordos Minoris everywhere - to the point that there's massive overlap (Such as an Ordo specifically for overseeing the Ecclesiarchy, which happily falls within the Hereticus' remit) and groups of fewer than ten Inquisitors (or in one case, a single Inquisitor) being called Ordos, entirely missing that half the point of an Ordo is to be a pool of resources and allies. (This is serious. I'll call my Ordo for help. Come here me!)