After the reformatting of UEFA-competitions in the 90s saw more teams playing in the European Cups and increased the number of games in those tournaments, top-leagues' teams changed the way they managed their squads. As most of the leagues now have 20 clubs, 44 matches per season is the lightest possible schedule for a team involved in European Cups, excluding domestic cup competition. Therefore, every team aiming to stay on top in the long run have to be equipped with a squad of at least 25 players, and preferably more than two for every role.
However, having a long squad list and making good use of rotation is a different thing altogether, as coaches also have different ways to manage their players. Rafa Benitez used to rotate his team very often when he was at Liverpool, and even if he never won the Premier League, he brought the club to two Champions League finals, winning one in 2005. On the other hand, Josè Mourinho always had a long squad list available both at Chelsea and Inter, but he made little use of rotation, as he never wanted to take the risk of dropping points, even when matches looked easy on paper.
Two cases are pretty interesting in Italy amongst top-teams that make less use of rotation: Juventus and Napoli have only used 24 and 25 players respectively so far, with just 18 of them having at least 5 appearances. Udinese also come close, but their small squad is the result of a different (and lower) budget available for team building compared to Juventus and Napoli.
After a number of unsuccessful seasons, Juventus have had the chance to play in the league without having to focus on European competitions. Therefore, rather than having a long roster of average players - as was the case in previous years - the club built a team with a few reserves but also some stars (like Pirlo) to give Conte the chance to fight for the title. The coach has made a good use of them so far, as AC Milan were only able to come back in February, when Juventus had a busy schedule due to Coppa Italia and re-arranged league matches.
In defence, Conte didn't have a reserve for Lichtsteiner until Caceres moved from Sevilla, and only De Ceglie could come in and allow Bonucci some time to rest by moving Chiellini in the middle of the defence. The international defender has missed just one game, while Barzagli has always been on the pitch. The coach did a very good job by moving Giaccherini (or even Pepe) into the midfield when Vidal was absent, as well as following the tactical evolution from 4-3-3 to 3-5-2 that made wingers less crucial. The attack is the only zone of the pitch where Conte applied rotation, adapting to different opponents.
If Juventus' manager had to build a brand new team, Walter Mazzarri could work with the players he had available in the previous season, with some reinforcements. Inler and Dzemaili came into the midfield, Fernandez and Fideleff joined the defence and forwards Pandev and Santana also signed. Mazzarri has already achieved a historic qualification to the Champions League knockout stage, beating 3-1 Chelsea at home in the first leg. Meanwhile, Napoli are still in the Coppa Italia semi-finals, and the race for the European spots in the league is far from over.
Despite this, the coach has kept his managerial philosophy, using his typical XI most of the time. The four-man midfield has six players battling to make the starting squad, but in defence and attack it is rare to see a change from the usual line-up. Unlike Conte, Mazzarri has more players to pick from, but every time he has tried to rotate the team, especially when going through a busy period in the calendar, they have dropped points in the league against smaller clubs. He is doing a great job by making his team perform as they do in big matches, but his limitations may be managing a bigger squad that could allow him - and his team - to aim even higher.
In the end, both coaches are doing very well, but both are facing the limits of their management: Mazzarri has to understand the benefits of rotation to let his team take the "giant leap" and compete for the title in the seasons to come. Meanwhile, Conte still has to make clear choices up front, or his forwards may begin to feel less trusted than the players in other roles and Juventus could continue to struggle to score despite possessing a steely defence. Now both managers have proven to be very good team-builders, it should be the time to take it to the next level.