Question

Is “either” only used with two options?

Answer 1

Not necessarily. One can you either to indicate a similarity to a group. For example:

'None of my friends are going to the party. I'm not gonna go either.'

or you can use either to compare multiple choices...

'She could go left, right or back home. Either way, it seemed Sarah would regret her decision in the morning.

Answer 2

Most commonly it is used with only 2, however it is sometimes used for more than 2 options. So basically, you can use it, but it's just uncommon and easier to omit the word completely.

For example:

You can have either 100, 200, or 300 bars of chocolate.
You can have 100, 200, or 300 bars of chocolate.

Whilst either works, it is unneeded.