«Збори» is a 'pluralia tantum' noun, it's always used in plural in this meaning. Just like 'trousers' can mean one pair of trousers or several of them, so «збори» can mean one meeting or several of them. If you need to distinguist them, you could add a numeral, for example.
Thank you. In that case an error report needs to be submitted, since the sentence "збори були довгі" can be parsed in two distinct ways, and therefore can be translated into English in at least two ways as well: "зобри [одні] були довгі" (a/the staff meeting was long) and "збори [дві або більше] були довгі" ([the] staff meetings were long).