There is a paper that gives an overview of the additive, subtractive and multiplicative derivations of number-words as used in the three dialects of Yoruba.
I'll quote a few things here, but the number system is far too complicated, and needs the paper I linked to to describe it.
In the first pattern, it uses addition solely to derive the number words but with a counting pattern where 'ten', 'twenty' and the added number units are taken as a whole. In the second counting pattern, subtraction is used to derive number words but this is applicable only to three numbers i. e. seventeen – /mɛ́ɛtadínóɡu/, eighteen – /méèʤìdínóɡu/ and nineteen – /mɔkɔdínóɡ́u/.
Most importantly, the idea expressed in [Standard Yoruba] and [Yoruba Ifè Nigeria] ... is 1 + 10, 2 + 10, 3 + 10, 4 + 10 for eleven to fourteen; and 20 - 5, 20 - 4, 20 - 3, 20 - 2, 20 - 1 for fifteen to nineteen; twenty (ogún) is a
The paper also compares the preferences of the older and newer generations of people when it comes to specific numbers.