Nice job, but I've always wondered, how many bullets does it hold?

@WHOZIT That depends on how you're asking. If you're asking with the mythos and lore of the show then the short answer is we don't really know. All we know is that within the lore of the show it is a Self Defense Engine Model B manufactured by Moses Brothers Firearm Company and it is the standard service issue pistol of the Independents army. It was not well liked by the Independents because it operates via a battery in the butt of the gun and that's what allows it to fire and reload (i'm assuming the battery never has to be recharged since there would be no way to do that). We also know that the gun is in fact an antique firearm and is no longer made. In one of the comments a villain that steals from Mal actually tries to steal Mal's gun under the claim that he "loves antiques". As to how many rounds does it carry nobody actually knows because they never got to it in the show or the film or even the comics. The gun has been heavily modified though by Mal and the way it appears in the show is the originally service issue however he modifies the Loaders in the comic Those Left Behind and then Mods it again in the film Serenity.

Now onto the Real Life answer. If you want to know how many rounds the gun Nathan Fillion carries I believe it's seven. The gun used in the show is actually a real gun. What they did was they took a Taurus (a .357 revolver I believe) and the prop department plated over it with metal plating to make the Model B. The gun Mal is carrying though is a real gun in the show I do not know about the one in Serenity though as that's made by an entirely different prop company.

For the size of the magazine in the gun, I would say 5-7 rounds, with maybe one in the chamber. But as the revolver was used in the real one, it would hold 6. But the way the gun is made and even shown when shot. It has no ejection port for the empty shell to be ejected from the gun. So it could never function as a semi auto pistol. If so you would see an empty shell flying out after every shot, so it could chamber the next round.
Unless, with the revolver part of it, after every 6 rounds you would have to remove the side plate, empty the shells and reload. But then why have a magazine? I love the gun(s) but it was made for looks

Well you posted the Atlas of the verse diagram for it and way I see it the ejection port is on the side of the gun.

I got the impression from the sound that its not firing normal bullets az we know & luv them; that it may not even be throwing solid projectilez.

Do you mean within the lore of the show or in actual life? In RL during filming it was shooting blanks (QMx did a very nice showcase of the real life prop on youtube). In the lore of the show it takes regular rounds. If you watch Serenity he puts regular good old fashioned bullets in the gun. The guns are powder weapons that fire bullets but they had to have their own unique sound because the show does take place 500 years into the future @JO753