If "hubo" is used for one item or many, when is "hubieron" used?

Hola Seth

Hubo is used to say "there was" or "there were" as explained.

This is when we use the verb haber in an impersonal way: there is, there was, there will be... etc

We also use the verb haber as part of perfect tenses: present perfect (I have worked), plupferfect (había trabajado), etc...

One of these perfect tenses is one called "pretérito anterior" or "antepretérito" which is formed by the verb haber in the preterite form and the past participle of the main verb, eg: yo hube trabajajo, tu hubiste trabajado, él hubo trabajado, nosotros hubimos trabajado, vosotros hubisteis trabajado, ellos hubieron trabajado. 

However, this tense nowadays in not really used anymore, as the pluperfect using "había..." took over and pretty much made the pretérito anterior an out-dated tense. 

I hope this clarified it.

Saludos cordiales

Felices fiestas

Inma