When do we use "廿", "卅" and "卌"?
As far as I know, in Mainland China:

廿 is only used in written lunar calendar. It is very rarely used in spoken Chinese. However, it may be used in some dialects such as Cantonese.
卅 is not used in spoken Chinese, except for representing the 30th day of a month in naming a few historical events and telegraph (popular in early 1900s but rarely used today).
卌 is nearly extinct and should be considered obsolete.

In conclusion: 二十 (&quot;two tens&quot;), 三十 (&quot;three tens&quot;) and 四十 (&quot;four tens&quot;) are enough.