According to Wikipedia:
Yan Tan Tethera […] is a sheep-counting system traditionally used by shepherds in Northern England and some other parts of Britain. The words are numbers taken from Brythonic Celtic languages such as Cumbric which had died out in most of Northern England by the sixth century, but they were commonly used for sheep counting and counting stitches in knitting until the Industrial Revolution, especially in the fells of the Lake District. Though most of these number systems fell out of use by the turn of the twentieth century, some are still in use.
The article gives over two dozen variants of the system, most of them with names for numbers up to 20. For example, the numbers 1 through 20 in Borrowdale (in the Lake District) go like this:
yan, tyan, tethera, methera, pimp, sethera, lethera, hovera, dovera, dick, yan-a-dick, tyan-a-dick, tethera-dick, methera-dick, bumfit, yan-a-bumfit, tyan-a-bumfit, tethera bumfit, methera bumfit, giggot
For a bit more about these and related terms, see the Wiktionary entry for tethera.