The Doctor has had 29 different companions since the program's launch in 1963. There are also many other characters who may or may not be classed as 'full-time' companions.
The Companions of Doctor Who was created in 1986.
They have all had multiple companions, apart from the eighth Doctor in the Doctor Who movie.
the most popular doctor who companions are Amelia Pond, Clara Oswald, and Rose Tyler
In the revived series (2005 to the current series), two companions have fallen in love with the Doctor. The first is Rose Tyler (she proclaims her love for him at the end of SE2EP13: Doomsday) and the second is Martha Jones.
The sixth Doctor's companions were Peri Brown and Melanie Bush.
You can find a list in the Wikipedia article "Companion (Doctor Who)". Decide for yourself which ones you want to include - some were with The Doctor only briefly.
The Doctor's companions in 2011 will be Amy and Rory.
The eleventh Doctor will star in The Sarah Jane Adventures along with the third Doctor's companion Jo Grant, apart from that there are no plans to bring any former companions back to Doctor Who.
The fifth Doctor (Peter Davison) traveled with three companions at once, Tegan, Nyssa and Adric from Castrovalva through to Earthshock.
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.
He had nine companions. Sarah Jane Smith Harry Sullivan Leela K-9 mark I K-9 mark II Romana Adric Nyssa Tegan
The Doctor's companions in Series 6 are Amy Pond, and Rory Williams.