How many currency types is too many? I'd say it depends on your theme and how much effort you're willing to force players to go through to learn your system.

If you're working in a world where currency can take electronic form, there's not much point in making your system confusing in an IC sense.

If you're working in a world like that of 200 years ago, yeah, you can have a lot of different kinds of currencies and even have moneychangers that take a bit off the top of the exchange rates.

Before that... you could use chunks of gold and silver bars that can be weighed and cut off as a medium of exchange. In other words, a pseudo-barter system where gold and/or some other items serve the functions of money*. Perhaps have a couple skills (if you're a code-heavy game) where haggling or a couple other skills can be used to 'manipulate' the metal or the perception of its value. Like with some gold, put a bit of silver inside so the gold seems more valuable than it is.

If you go even further back, you can have a barter system. Or if you aren't quite comfortable with a barter system have something that is considered valuable enough to be used in trade so that coded shopkeeping can be done.

In the end it depends what your objectives are, but if you're going to be heavy on the roleplaying and have a lot of other theme, a hard to understand money system serves as another barrier if you ask me. If you want that barrier it's great, if not maybe it's best to go with something more 'metric.'

* When I talk about the functions of money I'm talking about the economic definition, namely: Store of value, unit of account, and medium of exchange. - If you want, you could consider rocks a currency in your game.