Loads - in fact he's had too many. The list is almost endless, so I can't list them all here. His worst enemy is the Daleks, but there are also other enemies such as the Cybermen, the Zygons, the Sontarans, the Master and his less well-known enemies included aliens like the Slitheen and Judoon, who are much recent. The writers always add new enemies every other series or so, so it is impossible to count all of them.
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a lot as there are many new enemies and many old improved enemies in the confusion but if Doctor Who lasts as long as star trek then who knows how many there will be.
currently there are Daleks,cybermen,slitheen,racnoss,adipose,abzorbaloff etc but there are definitely more then 100.
The Silence, Judoon, Ood, Slitheen and Max Capricorn.
The general consensus is that his worst enemies are The Daleks- unlike most of his other enemies, they are unable to feel anything other than hatred, fear and a desire to exterminate all other life-forms, so it's impossible to reason with them or enable them to feel empathy or compassion. They are also virtually immune to most weaponry- only the most powerful laser cannon can detroy them, making them particularly dangerous.
Millions and millions!
The duration of The Best of Enemies is 1.73 hours.
As of today, (15/04/2017) 12 people have played the Doctor. This total can be raised to 13 if you count the War Doctor.