I don't know whether most countries forbid dual citizenship for their citizens, but as you say many certainly do; equally though many don't, and many others have varying degrees of proscription according to different circumstances.

There is a map here that someone has put together and that might be of interest. I can make no claims for its accuracy, of course, and in some cases which of the four categories a country ought to be assigned to must be a matter of opinion and judgement, since the situation is not always clear-cut. The map might also be out of date, since it has been around for some time.

In recent years there has been a trend towards relaxation of rules about dual/multiple nationality; whether this trend will continue, of course, no one can say for sure.