The Multi Refuel trainings enable a carrier to refuel up to 5 eligible ships at a time (all friendly ships with the exception of Carriers are eligible): The ship targeted, plus up to 4 other eligible ships in the squares adjoining the target on the North, South, East and West sides. But only if that Carrier has 4 or more Captain Skill points in Repair.
This is because the number of ships that a Carrier with Multi Refuel training can refuel in one shot, is limited to one plus the number of Captain Skill points that Carrier has in Repair. Therefore, since 4+1=5, as long as the Carrier has 4 or more Captain Skill points in Repair, it can refuel the maximum possible number of ships (5) at one time. Here's a crop from a screenshot of a Carrier with 4 Captain Skill points in Repair doing just that.
And here's a crop from a screenshot of a Carrier with only 3 Captain Skill points in Repair trying to do the same thing, but not quite succeeding.
Notice that only 4 ships got refueled in this attempt. The copper colored fighter in the square adjacent to the target on the East side ship got skipped because, having only 3 points in Repair, this Carrier can't refuel more than 4 ships (1+3=4) at a time.