"We had many cats."
Translation:Noi am avut multe pisici.
Because it is the past tense. Noi avem would be "we have". "Noi am avut" is "we had."
Present tense conjugations: Eu am Tu ai El/ea are noi avem voi aveti ei/ele au
Past tense conjugationsare formed using these parts of a avea followed by the past participle (e.g. avut) of the verb in question: Eu am tu ai el/ea a noi am voi ati ei/ele au
Therefore, Noi avem = we have whereas Noi am avut = We had
The conjugations of the auxillary verb 'a avea' used in the past tense are very simliar to those of the present tense. So I can see why you are confused.
Just pay attention to el/ea a, noi am and voi ati for past tense - the rest is the same as present, so only really 3 new words to learn