This is a great topic. Like you've suggested, there is no magical number out there that settles the reliability issue for all time. However, I tend to feel more confident in a new weapon once I've crossed the 1000 round mark. As for ammo, I like to start off as "hot" as possible with any new gun...for example, I fed my three month-old P226 Navy a steady diet of 9mm NATO-spec for the first three range trips. At first, she wouldn't function well with the more sedate commercial loads (i.e., Winchester "white box" Q4172, 115 gr. FMJ) and several horizontal stovepipe failures occured (5 total stovepipes with "weak" ammo during the first 300 rounds, no failures of any kind occurred with the "hotter" 9mm NATO ammo).
After running 350 total rounds of 9mm NATO through the new Sig, she got to the point that she'd function with any load I tried (including Win. Q4172, Remington UMC 115 gr., Sellier & Bellot 115 gr., Fiocchi 115 gr., etc.). Today, I just topped the 1200 round mark, and I haven't had any malfunctions since the few that occurred during the first 300 rounds through the gun.
I've also mixed in some JHP Ammo here and there with no feeding issues (100 total rounds total of Ranger-T 127 gr. +P+ and Federal HST 124 gr. +P). It probably helped that I waited until I had 500 rounds down-range before trying any JHP ammo.
Forget the chat-room hype, most new guns do require a break-in or wear-in period of several hundred rounds in order to settle down and perform. Getting the slide/frame, springs, extractor, bore, etc. "worn-in" a little seems to aid overall function.