How much do you need to decelerate in order to be captured into a stable lunar orbit from a Trans-Lunar Ejection or TLI-type trajectory?
Strictly speaking, the minimum ∆v is zero: from a ballistic capture trajectory, marginally stable lunar orbits at about 20,000 km altitude can be reached for zero or nearly-zero ∆v cost; an orbiter can then descend to a lower orbit at leisure using low-thrust high-efficiency ion engines. More generally, low energy transfers allow a tradeoff between travel time and insertion ∆v.
From an Earth-Moon Hohmann transfer, it takes about 680 m/s to reach a low lunar orbit at ~100 km altitude.
The Apollo missions started from a faster transfer orbit, so they required a little more ∆v; Apollo 11's initial LOI burn was 889 m/s to enter a 113 km pericynthion, 313.4 km apocynthion orbit.