How many quarter notes are in a eighth note triplet?

An eighth note triplet rhythm is 3 notes played in the space of 2 eighth notes. You may find it easier to think of the eighth note triplet as being 3 notes dividing a quarter note (since 1 quarter note = 2 eighth notes).

1 quarter note.

Explanation:

In binary time signatures (such as 2/4, 3/4, 4/4, 2/2, 3/2, 4/2), rhythm is written in groups of 2 or 4 notes (these are regular subdivisions of the beat in binary time signatures), whereas in ternary time signatures (such as 3/8, 6/8, 9/8, 12/8) rhythm is written in groups of 3 notes (these are regular subdivisions of the beat in ternary time signatures).

Tuplets (which are the irregular subdivisions of the beat in any time signature) vary depending on which kind of time signature you're working with. Triplets (groups of three notes) are irregular in binary time signatures, whereas duplets or quadruplets (groups of 2 or 4 notes)  are irregular in ternary time signatures: there are also tuplets that are irregular for both types of time signatures (such as quintuplets, which are groups of 5 notes).

Triplets are formed by three notes of any kind that are subdivisions of a note of higher value. For example, an 8th note triplet will have the same value as a quarter note, whereas a quarter note triplet will have the same value as a half note, a 16th note triplet will have the same value as an 8th note, and so on... You could also say that triplets have the same value as a regular pair of the same type of note (8th note triplet equals two 8th notes, a half note triplet equals two half notes, etc...).

Hope this helped, cheers!