Does Australia limit immigration from any countries?

Australia's migration assessment is merit- and eligibility-based, but they may have a cap for certain types of visas and total intake of immigrants per year. To the best of my knowledge, their quota system is not allocated based on per country. It is based on type of visa category per year.

To my knowledge, Australia does not exclude any person from the investor visas. The limitation of the program is more about: whether you can provide adequate evidence according to AU requirements; whether you can meet application deadlines; whether you can satisfy different authorities and English, health and character exams. Thought the above may sound easy on each of its own merit, the complexity is multiplied if you are not familiar with the AU legal system. Hence, it is highly advised for you to use a registered migration agent or an immigration lawyer for your application.

No, Australia doesn't set limitations on migrants from each country. Certain high-risk countries are subject to thorough background checks.

Australia does not have restrictions or quotas by country for visa programs. There are some specific visas that are made available to certain countries through bi-lateral agreements, such as working holiday visas, and some requirements for visas, such as employee-related visas, that are affected by inter-country agreements like free trade agreements. These serve to make additional options available rather than enforce restrictions. There are character and security background checks performed for most visa applications and these often can delay processing for citizens of "higher risk" countries such as Iran, but this delays only and does not affect eligibility (assuming there are no adverse findings).

No, Australia does not discriminate.