Parallelism, the “correspondence, in sense or construction, of successive clauses or passages” (OED). Benvolio compares his own mood and behaviour with Romeo’s in a pair of parallel phrases: “pursued my humor not pursuing his” and “gladly shunned who gladly fled”.
Antithesis, “the juxtaposition of contrasting ideas in balanced or parallel words”. Here we find “his affections” contrasting with “my own”; “most sought” contrasting with “most might not be found”; “pursued my humour” contrasting with “pursuing his”, and “gladly shunned” contrasting with “gladly fled”.
Isocolon, “a rhetorical scheme in which parallel elements possess the same number of words”. In this passage we find “his affections”—“my own” and “gladly shunned”—“gladly fled”.
Schesis onomaton, the “emphasis of an idea by repeating it rapidly using slightly different words”. Here the lines “pursued my humor not pursuing his” and “gladly shunned who gladly fled from me” repeat the same idea (that Benvolio and Romeo have been avoiding each other) in different words.
Paradox, “an apparently absurd or self-contradictory statement or proposition” (OED). Here there is an apparent self-contradiction in Benvolio’s claim that he is “one too many” even though he is “by my weary self”.
Amphiboly, “a sentence that may be interpreted in more than one way due to ambiguous structure”. Here the participial phrase “being one too many by my weary self” modifies either the line above, or to the line below, there being no way to determine which. (Shakespeare often employs this device; see the discussion of it in William Empson’s Seven Types of Ambiguity, pp. 50–53.)
Antanaclasis, “the repeated use of the same word, each time with a different meaning”. Here, in “most sought, where most might not be found”, the first occurrence of “most” is an adverb meaning “to the greatest extent”, while the second is a noun meaning “the greatest amount or quantity”.