A ton, most likely, with only a few (if any) alt-currencies surviving. I suspect that a currency with a slower hashing algorithm (even slower than Litecoin's) and a lower overall difficulty might be able to survive, if only as a conversion currency, due to the fact that more people would be able to make a profit for a longer amount of time. The main weakness I find with any of them right now is that, for the average person (and those like myself who "get" cryptocurrency, but are reluctant to sink fiat into it), there is no chance of one's mining hours being profitable over the long term, which means that after a certain point, you *have* to literally buy into the system in order to use it. For the very conservative among us, that is too risky an investment, especially in a system where the "exchange rate" amounts to "whatever value you think it has," which can wildly fluctuate because it isn't really based on commodity value or on "previously accepted value."
It doesn't help that the exchanges have almost no accountability, and how they set prices is clouded by murk and shadow.
From my perspective, right now cryptocurrency is really an investment trying to be a currency. Don't get me wrong, I think cryptocurrency has huge potential as "the cash of the digital world," but it needs to mature a *lot* before that can happen.