@Poli: You will certainly find "habían" everywhere, but "había" is the grammatically correct conjugation, because it's the imperfect of "hay" (an impersonal formula).
However, since this particular sentence is a guess, it would be better to say "
habrían sido cincuenta o sesenta hombres". And in this case, "haber" agrees with the plural, because it's a compound tense.
When you say "había cincuenta o sesenta hombres disparando", you are fair certain about the number of men shooting. If you say "serían unos cincuenta o sesenta", you're talking about your own impression at the moment, which might not be accurate.
@Tyrn: "Serían" here, introduces a guess (about how many men were shooting), and this is daily language.
— ¿Cuántas personas entraron a la tienda? (How many people entered the store?)
— No sé. Serían unas tres o cuatro. (I don't know, maybe three or four.)
- De los nuevos restaurantes, tal vez serían unos cien los que cerraron. (From the new restaurants, maybe about one hundred closed down.)
"Ser", as a guess, is also used in other tenses:
- ¿Crees que será hoy el día que por fin terminemos el trabajo? (Do you think it will be today at last that we'll finish the job?)
- El ingeniero ya está en la cárcel, pero el total de construcciones derrumbadas será de unas treinta. (The engineer is already in jail, but the total amount of collapsed buildings must be about thirty.)
— ¿Hace cuánto vives en la ciudad? (How long have you been living in the city?)
— Serán unos veinte años. (Maybe some twenty years.)
- ¿Crees que salir a la calle habrá sido una buena idea? (Do you think that going out was a good idea?) -> We went out, but we reckon we shouldn't have.
- Oí un ruido muy fuerte. ¿Habrá sido el viento? (I heard a very loud noise. Could it have been the wind?)
- No creo que fuera tanta gente la que estuvo en el concierto. (I don't think there were so many people at the concert.)
And yes, "cincuenta o sesenta hombres" is the only way to say it.
If you say "cincuenta o sesenta de los hombres", you mean "
from the bigger group (let's say some one hundred men), fifty or sixty were shooting (the other forty weren't)", so there's more certainty about the number of men shooting.