def print_double(x):

Question

is x a placeholder for any number or is x a set argument in Python

Answer 1

As you wrote:

def print_double(x):

... 'x' is argument.
But if you write:

def func1(*x):

... then 'x' is a tupple of arguments passed to the function.
And by writing:

def func2(**x):

... you'll get a dict of nammed arguments.
In a similar way, you can do the inverse, to call a function requiring many arguments with list or dict, by prefixing them of one or a pair of asterisk ( star * )...

Answer 2

it is argument like other languages.

Answer 3

x is an argument..

def print_double(x):
x=x+x
print(x)

a=5

print_double(a)