To unlock all deployed fleet slots, you could do the following;
 
1. National Policy
- Despotism > Royal Bodyguards or Liberalism > Volunteer Army



2. VIP
- Level up your VIP to level 6. VIP points can be purchased from Exchange Center

To check your VIP points - Commander's Info > Tap this icon 



3. Research Project
- Research Facility > Fleet Research > Deployed Fleets



Note: Maximum number of deployed fleets = 5