“I, measuring his affections by my own, which then
most sought where
might not be found, being one too many by my weary self, pursued
my humor not pursuing his, and gladly shunned who gladly fled from
The quote is from Benvolio, who had previously told Romeo's mother that he had seen Romeo while out walking to clear his head --
A troubled mind drove me to walk abroad
So the 'which' refers to his own affections, i.e. how he was feeling, his mood/ troubled state of mind. In such a frame of mind his mood most (mostly or most of all) sought out a place where there were not too many people -- where most might not be found.
In short: 'which' refers to his affections; the first 'most' modifies
'sought' -- the (active past tense) verb in the relative clause -- and the second 'most' is a noun meaning 'most people'.
[Incidentally, the phrase might even be deliberately ambiguous as far as the meaning goes. Romeo at this point is in love with Rosalind who doesn't return his affections. Perhaps Benvolio is also secretly in love with someone who isn't returning his affections so his affections are seeking the affections of the loved-one, but in vain -- "then most sought where most might not be found". In that sense the second 'most' would also apply to affections.]