Is "art" a countable or uncountable noun? Also, is it singular? If so, is "arts" the plural form?
'Art'is a singular uncountable noun. It does not have any plural form.
'Art' is a singular, abstract noun, and as such it is uncountable (as love, taste etc.). Since it is uncountable, it does not have a plural form.
"Art" may be considered countable when it applies to different art form. In this case, the plural form: "arts", is acceptable.
When referring to the same form
For example, a gallery might have hundreds of paintings. Those paintings would be referred to both individually and collectively as "art" (singular form).
However, when we talk about different art forms, we use the term "arts". For example: sculpture, theater, music, painting are different art forms that may be collectively referred to as "the arts".