I have never used numbers, but always colour coded. The ewe and lamb being marked the same, with one mark for a single eg single line or circle just above the tail, or midway along back, or just below the neck. The line could be vertical or horizontal. That gives you 9 different marks for a single, with one colour alone. Given the ewe is marked the same, you could then tell by looking at the ewe how many lambs she has, and by looking at the lamb, whether it should have any siblings with it.
Twins would then be marked with 2 lines, or a line and a circle - a circle counting as a single mark, as it was done with one stroke. These could be all one colour or a combination of 2. The 2 lines could be parallel, (horizontal or vertical) or a cross, and again in any one of 3 places along the body and the ewe marked the same. With say 4 colours the combination was virtually infinite, or should you still need more there are plenty more colours available.
Similarly with triplets - any combination of colours, lines and circles that made up a count of 3.
I found this much easier than trying to get a legible number on a wriggling lamb, and the colours lasted longer than the numbers remained discernible. I also found it easier as mentioned, to be able to instantly see how many lambs a ewe should have.