Hi Rev,
I think that the number is dependent on the use. Bitcoin has a limited supply of 21m, many of which will be mined long after we're gone and this has been effective for the first cryptocurrency, particularly when you consider the satoshi/smaller units available.
For Ripple (XRP), there are 100 billion total, with a division of 20% to the founders, 30% for development funds, and 50% to distribute towards adoption. With xrp being destroyed with each transaction (as a safeguard against certain attacks), the large number of xrp makes sense, though all 100 billion are not in circulation.
I don't think there's a hard rule on how many coins is the right amount for the value to hold because there is such diversity in the crypto space and there are many ways to artificially modify both the value and the supply, intentionally and otherwise.
I agree with another reply, it's more important that the the amount in circulation is reasonable for the use and that the unit is divisible as the price grows relative to fiat (and adoption rates).