There is no limit on the amount of risks a project can have. Any risk that the project manager knows of while doing risk analysis on the project should be listed in the risk management plan.
Again, there is no maximum number of risks, and if your project has a millions risks, then so be it, but in this case, you might be better off thinking about dropping this project altogether. A project with too many risks is not good news for the organization (and the project manager). The higher the number of risks for a project, the higher the chance of failure. In other words, the number of risks is inversely proportional to the project's success (please note that this point is highly debatable, and many other project managers might not agree with it).