The Time-Space Administration Bureau of Mahou Shoujo Lyrical Nanoha oversees at least 97 "non-administrated" worlds, since Earth is number 97. The title of "non-administered" is distinct from "uninhabited", so we can assume that all of these worlds have some sort of population. The number of "administered" worlds is unknown, but is probably more than one. Depending on how accurately certain sources have been translated, the highest number I know of offhand is either 12 or 61.
Judging by events in the series, the Prime Directive is loosely enforced in the case of the non-administered worlds, and there is no injunction against revealing the existence of the TSAB when necessary.
It's a bit hard to decide the traits for a brand new character who's supposed to seem a bit "foreign". Foreign to where? Japanese elementary school children can converse perfectly well with magical artefacts speaking in English, and do battle complete with banter with magically-constructed beings from several thousand years ago, who wield weapons speaking in German. Later, all of them head to an alien world where everyone speaks Japanese. We may postulate the existence of translator microbes, or some sort of magical equivalent, but That Way Lies Madness, where the soft sci-fi of MSLN runs head-on into the hard sci-fi tendencies of fandom.
For the sake of our collective sanity, we'll leave out of our considerations the creator in-joke of naming almost every significant character after a motor vehicle or associated aspect. This does provide for some odd mental images when I see an ad for the Nissan Teana.
The core media of MSLN is the anime, and it clearly eschews physical, sociological, and anthropological barriers in order to tell a Cool Story. Later, the extra materials of the Sound Stages, the manga, the DVD informational booklets, and random creator interviews attempt to explain away the inconsistencies after the fact, but these usually raise more questions than they answer.
Thus far, I've had to completely discard two entire fanfic uberplots due to canonical incompatibilities that have, in the anime, all the emphasis of a passing mention, or a single medium-sized manga panel. (Indeed, the first attempt was shot down because of a single manga panel, which I had hitherto not seen before.) Altering the plot is inconceivable, quite literally; an odd quirk in the way I formulate stories means that these uberplots are extruded whole, a complete work requiring only filler text to be presented as a finished item. On the upside, everything is tightly-plotted, and the guns of Anton Chekhov sound off in perfect time. On the downside, it's all a house of cards, and I don't think that I should continue the bizarre metaphors any longer than necessary.
All of this has given me a sort of blase attitude towards writing MSLN fanfic. While I try to scrutinize every aspect of my Card Captor Sakura fanfics for inconsistencies, the ever-changing canon explanations for How Things Work in the Nanoha-verse has led me to fall back upon the default answer of "Because". I shall have Vivio and several friends attempt a school project, botch it horribly, and then try to hide the results from Grown-Ups, at least until Nanoha-mama comes home and wonders why Zafira has grown thirty times his usual size. Is Vivio even able to do this? Isn't she supervised closely by the authorities? Would her accidental mischief be allowed to go that far? What of the laws of conservation of mass? I'm certainly open to suggestions, but as long as I can present a given quantity of fun, regardless of what readers think afterwards, then I consider my mission well and truly accomplished.
And yes, I have been criticized for this already. Not my stories, but my belief in just letting things be. The slippery slope is brought up quite often, as well as accusations that I have completely done away with Common Sense. I have yet to truly grok why the debating techniques used are so… antagonistic.