A company retreat is exactly what it sounds like. Depending on the size of the company, you could be seeing the entire staff complement be present or you could have a situation where a subset of the company attends.
This often depends on the intent of the retreat and the stakeholders who will be necessary. For example, you often find that retreats are strategic at either the C-suite or departmental levels, which will mean that the people who you will likely see fall under one of those two categories.
With this in mind, you can't necessarily attribute a usual number of people to a company retreat.