You don't; you look it up in a table of nuclides. It's very difficult (as in, essentially impossible) to predict things like this.
There is 13 isotopes in the element boron. xD
copper has 2 stable isotopes
Seaborgium (Sg) has 17 isotopes - including isomers and metastable isotopes.
29
It has 10
Dubnium is an artificial chemical element and hasn't stable isotopes.
29
That depends on the isotope, as Argon has three different stable isotopes and many different radioactive isotopes.
Isotopes of the same element have different number of neutrons.
Polonium is the element in group 16 that has unstable isotopes. It is a radioactive element with no stable isotopes.
There is no limit as to how many isotopes an element can have. It can be given as many neutrons as possible with today's technology. As for stability and natural decay, that's another issue. Some isotopes can last for milliseconds, or less.
Isotopes of the same element have a different number of neutrons.