As a general rule it would appear that a newly minted rogue trader would only have one ship of variable ability, though presumptively, a Rogue Trader could start with two smaller ships (strictly by game rules, if you have enough SP and buy cheap enough ships...).

I can't point to too many specific instances off the top of my head, but certainly the RT books seems to suggest that well established dynasties and individual traders do, in fact, have fleets. I recall several times reading about the 'fleet of Rogue trader so and so did such and such', both in the RT books and in more canonical 40k works.  I may be imagining it, but in fact I thought I read something that suggested that down on their luck RT's may swear fealty to more powerful dynasts, adding their ships to the fleet of more powerful traders (in keeping with the feudal nature of 40k, this makes some sense).  

THis is isn't a threat to the Imperium in the least, as RTs generally do not have battleships at all (I've yet to see an official RT battleship hull, for example), and if they do its just the one. Warships are not as profitable to run, and so RT's tend to  have fewer dedicated combat ships... but they do have enough to operate as sort of a ready reserve for the local Battlefleet, to keep piracy to a minimum and so forth.

There is nothing in the works of 40k that I am aware of that says they don't build new ships.  Far from it.  The Battlefleet Koronos book has several new hull designs, and indicates that they are less popular because they are new.  It also suggests that 'destroyed' ships can, and are, salvageable, though the process is not much faster than building a new ship (decades to centuries apparently, though this seems abnormally slow, even for such a massive project. Yes, I am aware that they could very well be using big fucking rocks for hammers in some cases... but then again, you've potentially got 100k people available to work on assembly! That's a lot of big fucken rocks!)

Point of fact, most of the ships in Battlefleet Calixis are apparently only a few centuries older than the sector itself, excluding mostly the Grand Cruisers (which are, by default, as old as the Imperium)

Rogue Traders seem to be able to amass Fleets, through profit or adventure.
Though, trade routes may not be a "we pick up every 6 weeks" kind of venture; trade routes may mean a Rogue Trader leaves a sizeable admin unit behind, and comes back in 10 years for physical cargo.

Quote from: Ghost Whistler;479467What indeed constitutes the average isn't really the issue. It's understanding the practicalities. If a trader establishes a trade route to a previously known planet/race, how does that route manifest? Shipping must be involved and presumably to a point controlled by the trader.

It's a lot easier to pay someone else to haul your shit from planet to planet while you're dueling with Dark Eldar raiders on a volcano planet or whatever it is that Rogue Traders do on a normal thursday.

Quote from: FrankTrollman;479215Most chapters of space marines have one ship.

Although those warp ships also are more like carriers than galleons, because they launch other non-warp ships out of them when they arrive somewhere.

-Frank

It used to be the case that "modern" chapters (i.e. post-Horus Heresy) have only 1000 marines, right? So just having one ship makes sense.

Space Marine Chapters have fleets.  They might have one Battle Barge, which is a type of ship that outstrips even the huge Imperial Battleships. Some chapters have more than one of these giant ships.

However, they have lots of Strike Cruisers and smaller ships.  They are not exclusively crewed by Space Marines - they don't do that. Instead, they have command structures and crew that are similar to Navy and Mechanicus ships - their crew are their "bondsmen".

Rogue Traders aren't as powerful as Astartes Chapters.

Quote from: noisms;481075It used to be the case that "modern" chapters (i.e. post-Horus Heresy) have only 1000 marines, right? So just having one ship makes sense.

They have perhaps one core ship, a base of kinds (which is my understanding of a battle barge), but they will have many ships, even if they are smaller, because different marines will be on different missions or attending different duties.

Never mind that Space Marines are not infrequently shown to have 'small' warp capable ships with as few as ONE Space Marines on board (and presumably a comparatively puny bondsman crew).

A number of chapters are 'Fleet Based', including the Black Templars (on permanent Crusade since day one), and many more are based out of space stations (the Dark Angels and my personal fav (if much less well known) the Mortifactors, for examples).

Arguably the baseline for how powerful you are in space could be chalked up to wether or not you can have a major space station at your beck and call, not if you manage to order three or four ships around instead of just one.