Assuming there's space for the set screws to face perpendicular to the row of bars, the limit would be clear space between couplers adequate for the concrete to flow. Otherwise, your best move would be to contact a splice supplier for info on the dimensions of the couplers and tightening clearances.
If you have #18 bars long enough to need splices at spacing tight enough to be concerned about being able to tighten the set screws, I'd suggest looking at other options, such as prestessing strand or high strength threadbar.
Really depends on the couplers. I would argue you'd need at least max aggregate size between splice couplers at bare minimum. After that depends on coupler type. If they're screwed couplers then probably fine to stop there. If it's one of those locking bolted/screwed-type couplers then they'll want to make sure things get installed with the coupler facing outside so they can get their impact wrench in to tighten the bolts/screws.
May also make sense to stagger them to help alleviate congestion/consolidation issues.
And I'd personally try to avoid using #18s if at all possible. Those guys are huge and cost a huge premium.
Got my answer. #18 bars with Lenton splice couplers can be installed as close as 5" on center. We would not space them any closer than 7" on center. They are tightened with a standard pipe wrench to a torque specified by Lenton.
