How many K-9s are there? The original builder gave him to the Doctor (I think Leela insisted). I could be misremembering that. Could more have been built aside from the ones the Doctor built after leaving the first one with Leela? I know that K-9 worked well in terms of the actors rehearsing, but not as well with the metal dog on set.
We have the original one the Fourth Doctor and Leela picked up (was it around the 49th century?) on an Earth outpost. (Last seen on Gallifrey, probably a long time before the Time War)
The second one the Fourth Doctor built alone and eventually gave to Romana II when she stayed in E-space. (Last seen in E-space, though assumed to have returned to Galliffrey with Romana at some point prior to the Time War)
Then there is a third that somehow ended up as a gift to Sarah Jane Smith. It is assumed it was from the Fourth Doctor. (This one is known to be destroyed) - Could this have been the first one the Doctor collected after Leela died (assuming she died of old age at some point on Gallifrey).
Then the fourth (probably) built by the Tenth Doctor to replace the the third one as a second parting gift for Sarah Jane Smith. (Last seen active with Luke Smith)