As we all know in many online casino games, the number of raises is limited for each round.  In many of the online casino poker program the number of raises which one can make is limited to a maximum of three.
When a player is going to make a bet and raise, then that is considered to be raise number one.
After this raise the game continues for betting and if there are going to be players contributing to the raise, the player can again put in another bet, this is raise number two.
Then in the following round, if some other online casino player is going to make the bet and make one more raise then that is raise number three.
For any bet, there can be a total of 3 raises made, which can be by the same player or by different players in the table. Almost all online casino poker sites restrict the number of raises for each round to be three.
After the third raise, no body else in the table can initiate a fourth raise and the betting stops. And the game progresses to the next round accordingly.  In the showdown, the player who made the last raise and is still heads up is supposed to show his cards first.