Bit of anecdotal evidence here.
We see A319LR with 75500 MTOW and they are fitted with a single ACT (although have provision for the other 5)
DOH-ARN is 2492nm. 110 pax and bags and usually about 500kg cargo.
ARN-DOH we very rarely use the ACT. Fuel load varies from 16000 up to 19000 kg, and occassionaly more due to morning fog forecast in DOH.
However DOH-ARN regularly has fuel loads over the 19000kg mark. (Full fuel without ACT is about 19100kg). Arrival fuel in the winter about 3500kg.
If you wanted to regularly plan a longer flight, you would need a second ACT, but then there would be no room for the cargo, and maybe some of the bags! The holds now are loose loaded and chock full.