I don't think we can judge his best performance on just number of assists. He's had several games where he has made several great passes which came to nothing, and also some where his range is missing, that's just the nature of playing long passes.
For me he's had better games simply because he was defensively better. Part of his job is to provide passes, but no one is expecting Bonucci to be around the top of the assist column at the end of the season, he's a sweeper.