One argument for signing more players is now is that some players will not be with us next season (probably)
Greer (new covering central defender)
Zamora (new target man, of sorts, striker)
Wilson (new mobile striker)
If the players are available now they might not be in the summer and...
Transfer Towell to Attacking Midfield makes just the absence of cover at left back to be addressed (in numbers). Rosenoir is overly right-footed, which means stopping crosses and distribution is impaired at left back. OK for an occasional game.
We are not even deficient in strength in depth...
Derby have 24 first team squad and the injured Will Hughes (Under 21). But three left backs. Hughes and Forsyth are out for the season though, and Warnock is an older player.
Boro have only 23.
So our squad is not actually weak in terms of numbers. About 27. So if we sign anybody, I would...
Albion Injury hit 2013-14 (can't be any worse)
Over 20 appearances: 16 (one left the club)
10-20 apps: 9 (two left the club)
Under 10 apps: 8 (including loans and one leaving)
Answering the original questtion in a roundabout way.
In a good luck season, I thought we could get away with...
Leicester City 2013-14
Regular players 20
Occasional 8
Albion 2015-16
Regular: 17
Occasional 11 (excluding Sidwell)
and I thought we could manage with the 25 PL squad plus U21 youngsters. 18 regulars + 7 occasionals + our only U21 players Solly March, Forster-Caskey & Wilson.