What is the maximum number of characters in a currency symbol? In the program I am writing, I need to show a currency selector. I would like it to display currency symbols. For this, I need to know what is the longest, in characters, currency symbol (or rather what the maximum length can be, if there is any limitation).

E.g. USD, EUR, RUB all use single-character symbols: $, €, ₽. I initially assumed that it will always be one character. But I am obviously wrong, as e.g. Polish zloty symbol is zł.
Instead of currency symbols, some of which are used by more than one currency as @HotLicks suggests, you may want to consider the three-letter currency codes used in most currency exchanges. Here is a list: https://en.wikipedia.org/wiki/ISO_4217#Active_codes.