[@Greenie]
There isn't so much as a limit as there is experience and how much they learn and practice. To learn new forms of magic they need to expand their mana to adapt it to different types, so there is a limit to how much they can expand that pool, but that limit is vast, so the only real limit you should be concerned about is how much experience the witch has with each magic she knows. All witches are able to learn dark magic as a start though, that doesn't mean they will use it.

Magic items are also common amongst witches.