The numbers 50, 99 and 100 and 1000 recur constantly in Vedic texts.  The symbolic significance of this has to do with the mystic number 7.  Seven is the number of essential principles in Nature, and it the number of forms of divine consciousness in the world.  Seven multiplied by itself is 49.  The number one is significant because it is out of one that everything develops.  When 1 is added to 49 we have 50.  When 49 is doubled, and then 1 is added, we have 99, as in the 99 horses, rivers, and cities that are referred to in the texts.  When there is a double repetition of 49, and 1 added to both ends, the result it 1+49+49+1=100.  100 is a number of completion.