Season 8
As of 2017, season 8 remains notable for being the season with the most number of regular characters. Not only did it see a new companion in the form of Jo Grant, but it featured a regular enemy — The Master — who was the first regular Time Lord enemy of the Doctor to appear more than twice, and an expanded regular UNIT cast that now included Sergeant Benton and Mike Yates in most episodes. Adding in returning regulars Jon Pertwee and Nicholas Courtney, the regular cast swelled to six.
Episodes
1 Terror of the Autons (1) Jan 2, 1971 
2 Terror of the Autons (2) Jan 9, 1971 
3 Terror of the Autons (3) Jan 16, 1971 
4 Terror of the Autons (4) Jan 23, 1971 
5 The Mind of Evil (1) Jan 30, 1971 
6 The Mind of Evil (2) Feb 6, 1971 
7 The Mind of Evil (3) Feb 13, 1971 
8 The Mind of Evil (4) Feb 20, 1971 
9 The Mind of Evil (5) Feb 27, 1971 
10 The Mind of Evil (6) Mar 6, 1971 
11 The Claws of Axos (1) Mar 13, 1971 
12 The Claws of Axos (2) Mar 20, 1971 
13 The Claws of Axos (3) Mar 27, 1971 
14 The Claws of Axos (4) Apr 3, 1971 
15 Colony in Space (1) Apr 10, 1971 
16 Colony in Space (2) Apr 17, 1971 
17 Colony in Space (3) Apr 24, 1971 
18 Colony in Space (4) May 1, 1971 
19 Colony in Space (5) May 8, 1971 
20 Colony in Space (6) May 15, 1971 
21 The Dæmons (1) May 22, 1971 
22 The Dæmons (2) May 29, 1971 
23 The Dæmons (3) Jun 5, 1971 
24 The Dæmons (4) Jun 12, 1971 
25 The Dæmons (5) Jun 19, 1971