Even if 1/100th of 1% of the valid possible unique gate addresses actually had a gate in residence, there would still be over 4000 viable Stargates. Even if half of them are buried, and even if we visit one new planet every week, the show would be able to run and run for years yet!
Oh yeah - and the biggest thing which the above numbers forget to take into account... the calculation used a source of 38 glyphs. Yet we know that there are at least 40, if not 45, that we have actually seen. Which would of course send the total higher still. [1/100th of 1% of a 45-glyph system gives 12221 Stargates.]
However, we then get into the problem of unique P.O.O. glyphs which has been raised in another thread [so please leave discussions for there?].