Ok, so you plan to deploy your army. You've chosen to start two troop units in drop pods in your primary detachment, and one elite unit in a drop pod in your allied detachment.
Primary
------
HQ
HQ
Troop
Drop pod
Troop
Drop pod
Heavy
Ally
------
HQ
Troop
Elite
Drop pod
Heavy
You have 12 units in your army. Normally up to half (6) rounded up (still 6)of them can start in reserves. However units that must start the game in reserves, like drop pods, don't count towards the units in your army when calculating the number of units you can start in reserve.
Primary
------
HQ
HQ
Troop
Drop pod
Troop
Drop pod
Heavy
Ally
------
HQ
Troop
Elite
Drop pod
Heavy
Now you effectively have 9 units in your army and up to half (4.5) rounded up (5) of them can start in reserves. However units that start embarked inside units that must start in reserves must also start in reserves and also don't count towards the units in your army. (as supported by the
faq)
Primary
------
HQ
HQ
Troop
Drop pod
Troop
Drop pod
Heavy
Ally
------
HQ
Troop
Elite
Drop pod
Heavy
Now you effectively have 6 units in your army and up to half (3) rounded up (also 3) of them can start in reserves in addition to the units that must start in reserves.