I'm reasonably certain its 5's. Engineers are a fairly special case, they can't be split up. I just built an Engineering Corps transport vessel. Mostly engine and 25 engineering bays to move them in groups of five. Its civilian, which is a lifesaver for maintenance. For roleplaying purposes, it unloads slowly. No need for it to go so fast but the empires gettin' large these days.
Corps of Engineers Transport class Troop Transport 97,300 tons 956 Crew 4464.5 BP TCS 1946 TH 10000 EM 0
5138 km/s Armour 4-188 Shields 0-0 Sensors 1/1/0/0 Damage Control Rating 1 PPV 0
Maint Capacity 29 MSP Max Repair 100 MSP
Troop Capacity: 25 Battalions Cargo Handling Multiplier 5
Commercial ICF Drive (25) Power 400 Fuel Use 5% Signature 400 Armour 0 Exp 1%
Fuel Capacity 600,000 Litres Range 222.0 billion km (500 days at full power)
This design is classed as a Commercial Vessel for maintenance purposes