Hi Linda, You should use only "um" as a cardinal number. "Hum" can be used as an expression to indicate untrust, impatience or doubt. However, many years ago some brazilian accountants would use "hum milhão" (old portuguese) in checks in order to avoid fraud or forgery. This way, people would not be able to write anything before "um" to increase its value (such as "vinte e um"). Therefore, there is no reason for you to worry about any rules regarding the use of "hum" or "um", as the first form is no longer admitted. Hope this helps.

It's because if you write down in a check "um" (1) someone could change to "onze" (11) or another values. Hum also mean (Expression - I got it)