And it doesn't necessarily need to be boys, it could be all boys, or boys and girls, etc.
Do native speakers always use the pronoun when speaking informally? Wouldn't "tenemos gatos" imply "we have cats" from the conjugation?
'We have a cat' would be, 'Tenemos un gato'; 'We have many cats' would be, 'Tenemos muchos gatos.' This sentence can be translated literally: 'We have cats'.'
There's about six, they are all different conjugations of the word "tener", which means to have. (Present time) Here's a conjugation list if you want one.
Yo tengo = I have
Tú tienes = You have
Él/ella/usted tiene = He/she/you formal have
Nosotros/as tenemos = We have Vosotros/as tenéis* = You all have Ellos/ellas/ustedes tienen = They/you formal plural have
Or, if you prefer a link that goes into detail: http://www.spanishdict.com/conjugate/tener
*Used only in Spain
Ruby, I think you actually meant 'There are six'. 'Six', more than one, verb plural.