Hi Kushagra,
This is perfectly normal. Each egg takes a couple of days to form the shell, which is why there is a day between laying each egg. In order to increase odds of every egg being fertile, the pair will continue to mate until all of the eggs have been laid. With cockatiels this is usually up to 6 eggs. Many pairs will not begin to incubate the eggs until the 2nd or 3rd egg or even until all eggs are laid. However, once she is done laying eggs and they have started to incubate them, the male should not still try to mate with her. Normally they take turns sitting on the eggs – one during the day and one at night. If the male continues to want her to mate instead of helping with the eggs, then he is probably too young. A male cockatiel should be 18 – 24 months old before you let him breed and the female should be at least 2 years old.
Thank you for asking Lafeber,
Brenda