How many bytes of the last keystream block are used to XOR the final block of partial plaintext in CFB, OFB & CTR modes? I apologize if this is a dumb question but I'm trying to understand how the final block of partial plaintext is XORed using only n number of bytes in the final keystream block. 

How do we determine the number of bytes in the keystream that are required to produce a final block of the same size (128 bit) as the others?
Suppose a block is $b$ bits long, say $b = 128$, and the message is $\ell$ bits long, say $\ell = 187$.  Then the final plaintext block is $n = \ell - \lfloor\ell/b\rfloor \cdot b$ bits long.  So use $n$ bits of the final keystream block; in this example, $n = 59$.