It depends on where you got it from?? The Apple Store only has 8Gb 5c's in. Some of the mobile phone shops/networks have some old stock left with 16Gb or 32Gb available but are few and far between.
Your phone will tell you how much memory it has left.
Look and see how big the files are you want to download.
Keep in mind you should leave some memory left for the phone to work well. Youy can not fill it to the point you have no memory left or your phone will not work well.