If you are using a 5V supply - you must use 5V relay also, not 12V one.
Better not increase the capacity, but use a separate capacitor for each relay. You can still use just one main switch, make sure it's strong enough to withstand the sparks on discharging capacitors if you have many of them.
And you'll need a really powerful supply if you'll use many capacitors. And capacitor recharge times will affect the frequency of switchings you can achive.
You can add resistors to the capacitors and calculate the recharging time as per RC-circuit principle.