We brought in Llorente and Tevez last year, two starters.
The summer before that, Asamoah, Isla was supposed to at least challenge for a starting spot as well, and Pogba, though he wasn't brought in to start so I'm not sure if he counts here.
Three years ago, Pirlo and Vidal.
Getting more than two starters per season is unrealistic from a financial point of view.