You won't be needing much:
I used 5 small parsnips to get a nice medium bowl full of crisps but if you'll use the bigger ones, 2 of them will be enough, obviously depending on how many you want to make. Mine were quite small in the end but I preferred them that way. You can cut them into different shapes as well, make them for example lengthy and make a spiral before roasting.
Wash the parsnips. There's no need to peel them but you might want to remove any less nice bits with a knife. Cut them into thin slices. A cheese slicer is good for this or you can just cut them with a sharp knife.
Pour olive oil into a bowl. Give the parsnip slices a bath in there, so that they get greased from both sides. Then place them on a baking tray or an oven dish next to eachother. Ideally there should be no touching between them. Sprinkle sea salt over them aiming that each one will catch a few salt flakes.
Roast at about 200 C until they are crispy and golden brown around the edges. Lower the oven temperature if the crisps are starting to burn.
Tap the excess oil off with some kitchen roll and you're good to snack.