"which" is used to refer a noun and what is the noun here? use "slash and burn" procedure
Emily Dickinson's letters (we arnt talking about emily neither dickinson but about letters)
to Susan Huntington Dickinson....which were written

letters are plural...were is used after "which" so automatically we know that this "were" isnt for dickinson but for letters. "which were" is modifying letters. when ever in doubt, look for the verb usd after "which"

we are actually talking about letters, so main subject is letters not dickinson

i hope this helps

Hi Sana ,
Can u please tell me what is a "slash and burn" procedure ?

Thanks!

slash and burn procedure is to remove all prepositional phrases and modifying phrases
in the above example i remove "tosusan huntington dickson
Emily Dickinson's letters to Susan Huntington Dickinson, which were written [spoiler]over a period beginning a few years before Susan's marriage to Emily's brother and ending shortly before Emily's death in 1886,[/spoiler] outnumber her letters to anyone else. now read this simple sentence and u will understand that why using "which were" and "outnumber" is correct.

Emily Dickinson's letters to Susan Huntington Dickinson, which were written over a period beginning a few years before Susan's marriage to Emily's brother and ending shortly before Emily's death in 1886, outnumber

Remember we know that which modifies the noun before it.
Here "to Susan Dickinson" is a prepositional phrase and it cannot be modified by which, so the noun that "which" modifies is the " letters".So its Usage is fine.and also as Sana pointed out correctly you can check the verb after "which" in this case it is "were" .Thus we know that "which" is correctly modifying plural noun(letters) here.

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