"A number of different letters appear on the screen."
Translation:Várias letras diferentes aparecem na tela.
The equivalent in English "A number of letters appears on the screen" is wrong so this seems to be a difference between English and Portuguese usage. In Portuguese "a number of" must be considered singular. I looked for a good reference and all I could find easily was this: http://www.newtoncbraga.com.br/index.php/ingles-para-eletronica/2487-ing037.
The number of devices in this pack is twenty (O número de componentes neste pacote é vinte)
A number of devices are not included in this pack (um número de componentes não foi incluído neste pacote).
Observe que, diferentemente do português, em que a segunda forma se mantém no singular, no inglês, ela vai para o plural.
[Edit: This is more tricky than I thought see: http://www.duolingo.com/comment/1622536.]
As a native portuguese speacher who completed high school, I can say to you that, in this particular case, "número" may work like a collective noun. Knowing this and one verbal concordance rule which says that you can focus either on the collective noun or on what the collective noun reffers to (de componentes), you solve the problem. So, it can be both "um número de componentes não foi incluído neste pacote" or "um número de componentes não foram incluídos neste pacote".
Thanks! That is true for several collective nouns in English, but not "a number of" as far as I know. I started a separate discussion about this subject here: http://www.duolingo.com/comment/1622536. I'll pop a note there to say you have provided an answer. [Edit: Oh, I see you have posted there as well, thanks.]
No. That's what I was trying to explain in my answer to Whirrun. "Um número de letras" is singular and therefore the correct form of "aparecer" is the 3rd person singular "aparece". "A number of letters" is plural, though (see, for example: http://www.dailywritingtips.com/is-number-singular-or-plural/).