A payment of $700 at the end of every month into an account that pays 10% per annum, compounded monthly?

How do you find n?

Based solely on the info you provided, you can't find n. If you had another value, e.g. Future Value or Present Value, you could solve for n.

You look carefully at each letter in your sentence and I am sure you will find not one but multiple n

If you got an annuity account, depending on which type you got, you should get an illustration that shows you how much you get annually. The illustration only shows an estimate of an average percentage. You should look at your account monthly or contact your agent.

A payment of $700 at the end of every month into an account that pays 10% per annum, compounded monthly?

How do you find n?