Morris, there are lot of myths about that and you might hear stories that if you buy more than 2-3 players in season then sky will fall down but all that is just a myth...
The only what happen when you buy a new player is the tactic's familiarity drops slightly for every new sings player and the team cohesion also slightly drops but it's easily possible to bring the tactic’s familiarity and the team cohesion to the maximum level during the pre-season.
Just put the tactic into 3 slots and play about 7-10 friendly matches against weaker teams to guaranty big wins and that's all. Also, don't forget to set the match preparation to "Match Tactics" and the general training to "Team Cohesion" to speed up that process.
Here's my 1st season in EPL with Bristol City after promotion from Sky Bet Championship, I brought 15 new players to the club during the summer transfer window! Only just think 15 new players!
And here's how the table looks after 14 rounds
I'm using Instant Result + Eclipse V3 and if I used Vortex V4 and normaly played matches then the result might be even better...