There is some good guidance given by Oxford Dictionaries (OUP) here showing quite a number of examples.
The basic guidance is
Nouns ending in -o can add either -s or -es in the plural, and some
can be spelled either way.
As a general rule, most nouns ending in -o add -s to make the plural:
Those which have a vowel before the final -o always just add -s:
a list of the most common nouns ending in -o that are always spelled with -es in the plural:
singular plural
buffalo buffaloes
domino dominoes
echo echoes
embargo embargoes
hero heroes
mosquito mosquitoes
potato potatoes
tomato tomatoes
torpedo torpedoes
veto vetoes
some of the common nouns ending in -o that can be spelled with either -s or -es in the plural:
singular plural
banjo banjos or banjoes
cargo cargos or cargoes
flamingo flamingos or flamingoes
fresco frescos or frescoes
So there are some that 'require' the -es although there isn't a hard and fast 'rule' that you can apply.
Consider zero and hero; indistinguishable in their form and yet zeros is the standard plural of zero while heroes is the standard plural of hero.
"Plurals of nouns". Oxford Dictionaries. Oxford University Press. http://www.oxforddictionaries.com/words/plurals-of-nouns (accessed September 21, 2014).
Generally a proper noun uniquely identifies one person or thing. As such, different rules apply: it does not require an article, and cannot form a plural.
Cheese is uncountable, but we use the word cheeses. We are actually implicitly referring to muliple types of cheese, so it's not really a number of cheese- it's number of types of cheese.
In the same way, it is possible to make sentences that look like they have plurals of proper nouns, but really they are just a number of instances of something that bears that name. The thing that it is really plural- the instances- is implied. Here are some examples:
The Philippines -> the Philippine islands
Greeks -> Greek people
Johns -> people called John
You can also add an article:
a Safeway -> a Safeway Supermarket
an American - an American person
the Smiths -> family of people called Smith
the Appalachians -> the Appalachian mountains
Some people call these proper adjectives, but descriptions of what counts as a proper adjective, and how it can be used, vary a lot.
These is just one country that takes the definite article but isn't a plural of something else- the Gambia.
Best Answer
If you're referring to the idea in general, it's not countable. There's nothing to count. "Someone who practices magic", "friendship is magic", or "magic powers this device" would all fit this pattern.
However, if you're referring to some specific kind, it's countable. So "the magics of necromancy and enchantment" is legitimate: it's referring to two different but related things. (Similarly, "the peoples of England and Scotland" refers to each separate group of people, and "the monies allocated were spent primarily in two ways" does the same thing for "money".) This is not a very common usage, though, since for most purposes there's really no reason to make a distinction between different kinds of something that's imaginary anyway.
Or here's another example:
(Source: The Seventh Magic)