Try to get expected peak hour traffic volume. If only daily data available, use 10-15% as peak hour estimate. Then determine duration of entry procedure (fee payment, info forms to fill for site reservation, etc.) add in slow down allowance for vehicle stopping/starting up again, to get hourly pass thru. with hourly volume and pass thru, you have a good handle to estimate number of required bays.
It may also be advisable to assume equipment in entry booth(s) to fail at some time, and provide for a fall back booth.