It's entirely possible that the 12 generation limit was imposd by the timelords.  No Timelords, not limit.
If it's biological, I can see them coming up with any number of means to get around it. From the Eye of Harmony to having one of his clones return and merge with a dying doctor and trip off another cycle.
It's less a question of limits, and more a question of if and how they want to end the series.
	
		
			
		
		
	
				
			If it's biological, I can see them coming up with any number of means to get around it. From the Eye of Harmony to having one of his clones return and merge with a dying doctor and trip off another cycle.
It's less a question of limits, and more a question of if and how they want to end the series.
 
				 
 
		 
 
		
 
 
		 
 
		 
 
		 
 
		 
 
		
 
 
		 
 
		