There is a limit depending on the tables volume and hardward available. I had a table with 5 indexes being inserted at a rate of 700 records per second and the hardard couldn't keep up (With 4 indexes it could). But the same table with 10 inserts per second would be fine.
slimdave, I did think of mixed case after posting - thought that was in the domain of "b*gg*r*ng flies" - OK, with mixed case you get to around 10**54
(BTW if you're into trivia - Archimedes' "Sand Reckoner" is the earliest known discussion of naming big numbers. He estimated the number of grains of sand that would fit in a sphere the size of his "universe" - something like the size of the solar system. The biggest number in his first "period" was 10**80000 and he didn't stop there!)
grjohnson, what is this "hardware" you speak of that so limits human imagination?
"The power of instruction is seldom of much efficacy except in those happy dispositions where it is almost superfluous" - Gibbon, quoted by R.P.Feynman