does sett kingpin's passive "I get +1 might for each buffed friendly unit at my battlefield" works if im attacking a constedted battlefield along with my buffed units?

Yes. Sett's passive counts buffed friendly units wherever he currently is, attacking or defending — "my battlefield" just means the battlefield Sett is at (equivalent to "here").

Two things to keep in mind: he must actually be at a battlefield (no bonus while in base), and he only counts units at that specific battlefield, not your other units elsewhere. Sett also counts himself if he's buffed, so a lone buffed Sett attacking is 7 [M] — 5 base, +1 from the buff counter, +1 from his own ability (FAQ #7851, #2196). This is a passive ability, so it recalculates continuously rather than snapshotting; it updates as buffs are added or units arrive/leave.