How to choose instruments for GMM estimation?
As jmbejara says, better to be more specific, but let me guess.

You have a linear model $y = \beta_0 + \beta_1 x_1 + \cdots + \beta_{10} x_{10} + u$, where $x_1, x_2, \ldots, x_{10}$ are endogenous. Then, yes, you need at least ten instruments.

Can you use the same dependent variables as instruments for the regression? No, you can't. Instruments should be exogenous (uncorrelated with $u$) and relevant (strongly correlated with the endogenous regressors). But if some of the regressors are (believed to be) exogenous, then you use those exogenous ones as instruments. But you still need extra instruments for endogenous regressors. The rule is that you need at least $k$ extra instruments if there are $k$ endogenous regressors.

Finding good instruments (exogenous and relevant) is usually difficult. I suspect it would be really hard to find 10 instruments for a regression unless you have good economic justification.

That said, again it would be better to see your specific problem.