The standard number of regeneration cycles for a time lord is limited to 12 times (meaning 13 bodies). However it is possible for Time Lords to be granted extra regenerations. There is evidence the regeneration limit was something externally imposed by the Time Lords, and therefore may not exist anymore now that Gallifrey is gone.
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A Timelord can regenerate 12 times and have 13 bodies, the 13th time instead of regenerating he is supposed to die.
Time Lords regenerate because they have a injury or something is wrong, it's their way of healing themselves.
Trenzalore (his final resting place), in the town of Christmas
The eighth Doctor regenerated offscreen and nothing is known about how or why he regenerated. The eighth Doctor appeared onscreen only once in a TV movie. We know th Time War happened between the TV movie and the start of the 2005 series. It's widely assumed that whatever the Doctor did to end the Time War caused him to regenerate.
At the moment, The Doctor has regenerated 10 times, and Matt Smith is the 11th Doctor