Help with understanding isotopes?

"the proportion of each isotope of an element is fixed"- This means:

Potassium has 2 stable isotopes: K-39 and K - 41 . Because every K atom has 19 protons there must be some atoms that have 20 neutrons and some atoms that have 22 neutrons. You see that 20+19 = 39 and 20+22 = 22. You can work out the exact proportion of K39 to K41 but for purposes of this discussion let us say that for every 1000 K atoms you will have 999 K39 atoms and 1 K41 atom.

What this statement is telling is that if you get a sample of pure K from three widely distributed places , say Australia Russia and Mexico as examples - these three samples will all have the exact same ratio of K39 to K41. The ratio of isotopes is fixed or constant everywhere.

potassium is made of two isotopes: I have already discussed that K atoms all contain 19 protons , but some contain 20 neutrons and other atoms contain 22 neutrons . These are the two isotopes of potassium.

all elements have isotopes. isotopes differ from the periodic table element based upon the number of neutrons present. this means that isotopes differ in mass from one isotope to another. the more stable isotopes have fixed ratios of neutrons:protons. if an isotope has many more neutrons than protons, it is an unstable isotope. these ratios are called "magic numbers"

all elements have more than 1 isotope. when it states that K has 2 isotopes, this means that the predominant isotopes of K consist of 2 different isotopes. it doesn't mean that there are only 2 isotopes of K, just that the majority of K is made of these 2 isotopes.

the link on K isotopes below shows that there are more than just 2 K isotopes. this table also shows the 1/2 life of the isotopes and which are stable isotopes. as we can see from this, 39K and 41K are the 2 stable isotopes of potassium.

The proportion of each isotope in nature is at a fixed equilibrium. Potassium has only 2 isotopes that are stable; you wont find any others in nature.