At the top of the 500 Forum page is a 'Sticky' posted by 'Bambino' which is a copy of the Fiat 500L Owner's Handbook. According to page 57 of this pdf, the fluid capacity of the braking system is 0.22 litres.. Lots of useful info in this little manual, well worth a read.
Of course you'll need more than this amount of brake fluid if you are bleeding the brakes, probably need to purchase 500ml or possibly even more depending on how the bleeding operation goes.
P.S. Just in case you're not aware of this, don't shake the container of brake fluid, you'll just put air bubbles into the fluid.
AL.
