Here's a very loose upper bound: assume it takes two seconds on average to speak 3 digits and the appropriate -illion word, and that you can't keep speaking for over 72 hours without falling asleep and losing your place. Then, if you pick a number larger than about 10^(1000000), you can be absolutely certain it hasn't been spoken. I'm sure there are weirdos out there who could have spoken aloud all numbers between 716 and 717 million, for fun, for all I know, and maybe a group of such people distributed over human history has patched up any "holes" up to a large number, but if a single number is too large to speak of uninterrupted, that's sufficient.