5 gallons per wash, about 5 minutes, so a gallon a minute barely keeps up if your capable of lining up cars all day. This is when you will have a problem!
Also you may find you use slightly more than that if its the manufactures stated usage.. You may choose to run a pass slower, increasing the usage.
Ideally you have enough capacity to let the ro system rest when your at peak. Sure you can use a larger holding tank but still the system may run for hours. Space available will be a factor.
A two membrane in series system is more efficient on water and electricity, you double filter and get double the output with the same amount of power. Also may provide lower TDS levels at higher outputs over time because you won't try to max out the pressures to keep up, which will start fouling the membrane. Keep the pressure to 4040 film tech type membranes around 200 not much more.
If the single membrane starts to foul and you get a half or 3/4 gallon per minute you'll have a problem. With two you'd still get a gallon of output at this point.
I have 3ss with spot free, and one ryko touch less using ro for all chemical passes, and a 250g holding tank. Im using a 4 membrane serial system, added 2 more to my original system.. It really didn't become problem free till I put the forth one in 2 years ago... Now it keeps up no problem putting out 4-5 gpm with a tds of 4, using 3 old membranes that were previously fouled and cleaned, one new forth membrane. .