Rather than looking at it as spaces we need to fill its more a limitation of foreign players.
For the CL as Billy has said you can have 17 foreign, 4 HG & 4 Club trained.
We however don't have to fill all the spaces and could leave them blank providing we don't go over the 17 foreign players limit.
At present we have 15 foreign (Willy, Mangala, Sagna, Demi, Kolarov, Kompany, Zaba, Fernandinho, Fernando, Nasri, Navas, Silva, Yaya, Aguero & Bony) so that leaves us 2 free spots.
HG we have Clichy, Hart & Delph as Sterling being under 21 can go on the B list and doesn't have to be included in the main squad.