Learn Portuguese (Portugal): De repente, houve um tumulto na assembleia. - Suddenly, there was an uproar in the assembly.

This is the core difference between the two past tenses! We use houve (the Pretérito Perfeito, or simple past) because the phrase de repente ("suddenly") tells us this was a single, abrupt event. If you wanted to describe an ongoing background situation (like "There was an uproar going on for hours"), you would use havia (the Pretérito Imperfeito).

If there were multiple uproars, would houve change to a plural form?

No, it stays exactly the same. When haver is used to mean "there is" or "there are", it acts as an impersonal verb and only ever takes the third-person singular form. So, "there were multiple uproars" would still be houve vários tumultos.

Could I use foi or esteve here, since they also mean "was" in the past?

No. While foi and esteve are the past forms of the verbs for "to be" (ser and estar), they do not mean "there was". To express that something exists or that an event happened, Portuguese strictly uses haver (which becomes houve or havia in the past).

Is de repente a fixed expression?