Clattenburg has reffed them 4 times this season.
http://www.premierleague.com/en-gb/referees/profile.appointments.html/mark-clattenburg
I think Dowd, Webb and Atkinson have all done 3 each.
This season Dowd has reffed them 3 times, 2 wins 1 draw, the Derby. Last season Dowd reffed them 3 times 1 win, 2 draws.
Webb this season has reffed them 3 times in the league 1 win 2 lost and 1 cup game, lost.
Last season he reffed them 5 times 1 win, 1 draw and 3 lost.
Clattenburg has reffed them more this season than any other since 2007. He gets 2 games per season since the infamous Derby, this season as you say 4 times. What is unusual though is for a referee to ref the same team 3 times within 7 weeks. The usual rule of thumb is that a referee doesn't ref the same team within 6 weeks.
Webb got the Chelsea league game followed by the Arsenal cup game. He hasn't refereed them since.
Conte also claimed Clattenburg had failed to control what he saw as Benfica's spoiling tactics.
"The referee allowed them to obstruct the play the whole game. Before the game he told is us that he wouldn't stop play every time a player hit the floor, but that's what happened. We feel like we're not being taken seriously, and Uefa should show us more respect by sending a referee of the required standard."
Fun how it takes an Italian to question the ability of Clattenburg and not the media in this country.