Last season i signed 15 new players the day the window opened and followed that up with 3 more new signings, I'm in 2019/20 now so 2018/19 season, Not a single drop in morale or through the entire team, That being said those first 15 were all under 17 and purely regens for my under 18's, You can sign as many players as you like aslong as your team dont see them as a threat to their status in the team.
I've signed 10+ players before all on frees for resale later on but aslong as you have them signed as backup or rotation players it shouldnt upset the team, Signing 10 first team or key players is going to really unsettle the squad but rotation and backup players are fine and youths you can go crazy with.
My starting 11 has been the same pretty much for 2 seasons now and in that time i've had 41 players signed, All signed as backup/rotation and were sold off or are under 18's, Never been a drop in morale, Won the league both times, Champions League both times, FA Cup once and Capital one Cup once, As far as key players yes i would say 3-4 is a good number.