Sri Lanka's casinos are among the very few that are legal in South Asia. In fact, the whole country is rather more laid back and unstuffy about such things than anywhere else in the region. This is probably an influence of the majority religion, Buddhism, which pays much less attention to what other people ought to do than to what the individual might want to think about doing. That isn't something all that common amongst religions.
While gambling and casinos are legal all over the island, Sri Lanka's casinos are in just two cities. One in Kollupitiya and the others are all in Colombo.
A list of Sri Lanka's casinos includes:
Kollupitiya: Kollupitiya Casino
Star Dust Club
As can be seen from the names and their similarities with those in Las Vegas and elsewhere, these are not just local outposts. They are clean, properly functioning, reliable casinos.
There are probably many more casinos across the island, but we're not really able to find out quite how many. For only those with a casino license may call themselves a casino, while almost anybody can run a game of chance. So the true number of Sri Lanka's casinos is either unknown to us or, if you prefer, we know that nine call themselves casinos so that is the number there are.
Sri Lanka, the island, is a tropical near paradise (along with all of the implications of that including a separatists war in the north). Formerly known as Ceylon, it was a British colony and also has had substantial Dutch and Portuguese influences. The last added a dose of Catholicism to the islands religious mixture, the Dutch a bit of Protestantism and the Brits, in their way, didn't really take religion very seriously at all. There are also substantial Muslim and Hindu minorities.
One of the odder leftovers of that colonial history is in the cuisine. While the British took curry and tea away from the islands, you can still see traditional foods such as Bolo Fiado, a Portuguese style layer cake, Breuhder, Dutch Christmas Cake, and other influences maintained by the Burgher People (a long standing cultural rather than racial grouping, descended from intermarriages between the various colonial invaders and the locals).
With the multiple ethnic groups and religions on the island, and despite the one large rebellion in the North, Sri Lanka is, in general, an accommodating and relaxed place. There is certainly no outcry to insist that people should stop doing what they wish to do, as long as it harms no one else, which may explain the number and popularity of Sri Lanka's casinos.