In Hong Kong herbal tea shops, you will see a tea called "廿四味" (24 flavours)
(you will also hear "廿蚊" to mean "20 dollars" in small shops)
Are there any other chinese numerals that represent unusual numbers?
halfelf's answer is correct, but I'll say a little more about how they're used in Cantonese, since the behaviour of these abbreviations is a little unusual there.
廿 jaa6 indeed abbreviates 二十 ji6 sap6. Similarly, 卅 abbreviates 三十 saam1 sap6. But 卅 is pronounced saa1aa6, an unusual tone contour.
Abbreviations for numbers 40 and above are formed regularly: sei3aa6, ng5aa6 and so on.