We should really consider getting a CM. Busquets, FDJ, MP, Puig, and Alena are not enough. Would not hurt to reinforce the team since we are allowed to use 5 subs.
I think 5 players are more than enough for 2 positions if we'll stay with a current formation. We also have Ilaix in Barca B who could play in Copa or maybe some easier league games and Roberto can also be used there. I think if Dest comes Koeman will trust him more than people believe and he could become a starter in a few months so this would allow us to play Roberto in midfield more. But I think he would play in the FDJ role (giving him rest) more than in the Pjanić/Busi one so it might eventually cost Puig more minutes which some people won't like.
The way I think Koeman wants this team to play is by having one more static midfielder to keep his position in midfield (Pjanic/Busi) while the other of the two is more mobile (FDJ/Puig, could also be Roberto). Now if we'd need to score then we could turn to FDJ+Puig combination in second half but I don't see those two starting together often if at all. Alena is just a 5th/maybe even 6th midfielder who will play Copa and get "garbage time minutes" here and there unless we'll have injuries/suspensions problems. Or if he'll use his given minutes extremely well (I doubt) and push Koeman to play him more.
Now if we could get a midfielder who would really improve the team for a good price and (more importantly imo) bring something different we currently don't have in midfield I would be all for getting him.
Wijnaldum was linked to us but I see him more as an AMC where we already have Messi/Griezmann and Coutinho/Pedri (and if Depay comes he can play there as well). I don't see him fitting in the MP/Busi role or as a sub for FDJ. But if Koeman has different plans and we could get him for around 15m (and not longer than a 3-year contract with possible club optional extension for another season) then I wouldn't mind him.