Definitely. Yu-Gi-Oh began as a manga series and its success led to the creation of other related media such as the anime and the card game. The manga laid the foundation for the entire franchise.
Sure. For example, there might be variations based on different creative teams or targeted at different audiences, resulting in two separate Yu Gi Oh manga.
The total number of Yu-Gi-Oh manga volumes is not fixed and can change depending on how it's packaged and released. Generally, though, there are quite a few, likely more than 30.