There are many kinds of sea urchins and they populate very different environments, from the inter-tidal to the deep sea. Needless to say, these different urchins can have very different life histories.
Most sea urchins are mass spawners, producing several million eggs.
If even a tiny percentage are fertilized, you will have thousands of live offspring. And only a small percentage of these survive in their planktonic form without starving or being eaten until they find a place to settle as tiny adult urchins.