Squire's Shield
Increase your max number of sentries by 1 Increases summon damage by 10%
3Dropped by
what it actually gives
An extra sentry slot plus a flat summon damage boost is a heavy package for the point in the game it drops. Two sentries out instead of one roughly doubles your stationary contribution during the Old One's Army, and the damage bonus carries over to minions as well, so a summoner gets value from it in every fight rather than only in the event.
do not wear two
The Old One's Army trinkets do not stack with each other, in sentry count or in the damage bonus. Wearing a second one is a dead slot. Pick the one whose damage type matches your build and store the rest.
worth a slot even without minions
Offensive accessories are thin before Hardmode, so even a player who never summons anything gets free background damage from a sentry parked in an arena. It also counts as a shield for the purposes of the Brand of the Inferno, so it enables a manual block on a build that otherwise had none. Note that it grants no defense of its own, which is unusual for a shield and will catch you out if you were budgeting for the usual bump.