India as No.1 can't be taken seriously, of course. No one can doubt that they have improved in the last few years, but until they at least draw a series in SA or Australia and win outside their country consistently, they can't have the title. SA had 12 unbeaten consecutive series before they lost recently to Australia. If India can manage something similar to that over a period of time, I think they deserve the title.
I'll take your number of 12 series and compare last 12 series of Ind & SA.
Indian record:India has 3 losses, 1 draw, 8 wins. It lost no home series and away losses have been to Aus,SL,SA and in each case the series score line has been 2-1. (Without an incompetent umpire in Sydney, this would've been only 2 series losses in the last 12).
Let me restate that really carefully: There has not been a single home or away series in the last 12 that India did not win a test in, and not a single series that it lost by a margin of more than one loss. (Aus or SA cannot claim this).
Away wins have been Bang, WI, Eng and NZ (which was the real bugaboo of the Indian team due to 2002).
SA record: 2 losses, 1 draw, 9 wins. (but they played Ban, Pak twice where India played them once)
SA just lost a home series to Aus. Plus they lost away to SL.
SA won in Aus (which is very creditable) but the quality of their other away wins is not that different Pak, Bang, Eng.
Let me restate one other quality of series win really carefully: SA has shut out (ie conceded no games to) Bang and NZ, thats it.
India has shut out Aus, SL, Eng, WI besides Bang, NZ.
(Australia have won 9 and lost 3 btw).