There have been many assistants within the show's run, and even more yet to come unless you only call the classic assistants "assistants". The first assistants, as you term them, may have been schoolteachers Ian and Barbara who traveled with the first Doctor and his granddaughter Susan. They helped bring out the Doctor's more compassionate side to include more than just his family and had influence on his preference for humans as his favorite alien species.
If this answer were to include the most recent companions, it would probably make it a bit dated in reference. However, the Doctor has aided and been aided by many, many allies and will continue to be as long as the show is still made.
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It depends. If you count the first two K9s as two separate companions, then the first and fourth Doctors tie for 9 companions each (not counting characters who were only traveling with the Doctor for one story). Otherwise, it's the First Doctor.
Including the newest one, there have been 11 incarnations of The Doctor, though there have been 12 actors. The First Doctor was recast for his appearance in "The Five Doctors" due to the original actor's death. So, there have been 12 actors, portraying 11 different incarnations.
Colin Baker played the Doctor in 1993; however, the Doctors name is literally always the Doctor.
11 i think
Her main Doctors were 3rd (Jon Pertwee) and 4th (Tom Baker), she appeared on 'The 5 Doctors' where Doctors 1 to 5 met together and then she saw 10th Doctor (David Tennant) and then met the 11 Doctor (Matt Smith) on Sarah Jane Adventures This shows that she was involved with Doctors 1,2,3,4,5,10 and 11 (7 altogether)