Yes, well.....no.....well......yes.......ummmmmmm
Depends on the situation, the players, and the predictions.
Personally, I have found the scouted players list too short when starting at a new club to confidently make many changes to the squad - so maybe only a couple in the summer and one or two more in January if needed.
However, if you have a poor squad, lazy/old/can't be bothered players then yep, get rid as soon as possible, replace quickly and hope for the best - don;t expect quick success though.
In this version, and I haven't run a test or anything just observation, it takes up to 10 games for a team to be totally comfortable in any new set up/tactic without any imports. Wholesale changes to the playing staff will make this period longer as there will be a settling in period for the newbies so big changes in summer and it will be near Christmas before full familiarity is achieved - this does impact results.
Also, bear in mind the 'Dynamics' in FM 18 - lots of new players in and old favourites out will upset the dynamics and this too will impact results. However, sometimes you gotta make the tough decisions.
I am 10 games from the end of my first season in charge of Morecambe, and am looking at one First Team signing for MR to be set for my 'Defensive' formation (looks like we are favorites for promotion so need a Defensive tactic for next season)
Personally, when taking on a new club, I build a tactic based on what is immediately available in the squad, strengthen a couple of positions and go from there. Over the course of the first season (usually by around Christmas), I will have a better idea of who is available in the market, who I want to keep and what tactic I will be using going forward and look to bring in players to achieve that gradually.
Hope that helps