4.2.1 Fusion of undefinite article and preposition
The plural form of the undefinite article in english is some and has therefore nothing to do with a. (a man => some men). In Portuguese the plural form of the undefinite article is based on the singular form of the undefinite article. (um => uns, uma => umas). The singular form as well as the plural form are used if the noun is unknown (A man crosses the street = Someone unkwown crosses the street => The man crosses the street => A specific man crosses the street) or if the amount of elements are unknown (Some men crosses the street = More than one unknown men crosses the street).
The undefinite article has to agree in gender and number with the noun it refers to.
*note: The plural form of um is uns. Actually any singular noun ending in um forms the plural with uns.
The undefinite articles fuse only, in opposite to the definite articles / personal pronouns, see 4.1.1, with the preposition de and em.
I am waiting for some persons.
* * estar a esperar is a variation of the construction in Brasilian Portuguese of estar + gerundio what corresponds to the English construction to be + gerund. We don't care about that right now. We have a detailed look on the gerundio at 18.1.2 estar + gerundio.
* In this case the duma (de + uma) is not a preposition, but a genitive, but the logic is the same. De fuses with the indefinite article. As a genitive de corresponds to of.