The registration process began 4pm on Saturday, April 28, and the government has announced that over 1.32 crore people registered in just one day. There were initial glitches, probably because the portal was handling more than 50,000 API calls per second.

source https://gadgets.ndtv.com/apps/news/1-32-crore-registrations-in-one-day-cowin-portal-appointments-slot-state-governments-decision-all-details-2424248#rss-gadgets-all