(note that the noun is nearly always plural, castra, even when talking about a single camp).

There is no average number, because the castra could be made larger or smaller depending on the size of the force, local topography, presence and amount of impedimenta, and time of construction. Its basic elements were modular and expandable. If you could somehow list every Roman castra ever created in a spreadsheet, along with the size of the force sheltered within, I suspect the average would be about two legions, a regular consular army, about 6,000 - 8,000 men.