How do Greeks distinguish using Greek letters as a variable, or as a mathematical number such as Pi?
Pretty much every Greek letter is used as a mathematical variable. How would you differentiate between using a Greek letter as a specific variable (Such as μ for apple, as μήλο means apple) or as a general variable like Theta (The known degree of a right triangle) or Pi (3.14159265359)?
It's not much different in English.
r is the radius, i is the square root of minus 1, x, y, z are often variables, and so on. Single letters are used a fair bit in maths and physics. Yet people usually don't get confused.
It's usual that some letters are used for certain variables. θ,φ for angles, χ,ψ,ζ for unknown variables of equations, the constant π=3,14159 , ρ=radius etc. It's not common that you need all those variables in every problem, though. So, if you don't deal with angles at all, for example, you can use θ or φ for whatever you specify. Θ in physics for example can mean temperature (and not angle, of course). I hope I answered the question well, I am not sure if this is what you meant. ;)