OK. So the Scottish Register of Tartans says they have 13,000+ tartans. (they get 10 or so new ones every week on average) When they started back in 2009 they used the lists from the Scottish Tartans Society, the Scottish Tartans Authority, and the Scottish Tartans World Register. These, as you say figheadair, are usually said to be around 7000 since the 3 prior lists had to be combined which was not an easy thing to do because each prior list used different criteria. Then the Scottish Register of Tartans began their numbering system at 10,000.

My current search on the SRT has the numbers from 1 to 6,276 for the tartans gotten from the 3 original lists. Then there is a gap of 3,724 numbers until they restart at 10,000. So they have added 3,000 tartans since then. Which would be 6,276 + 3000 = 9,276. (as of this week).