You can deposit any amount of cash under $10,000.00 before the bank is required to complete an 8300 form which is used to inform the IRS of the cash deposit.
If you deposit $10,000.00 or more in cash or checks then the bank is required to report the deposit to the IRS.
Also if you make 2 deposits of $5,000.00 within the same week then the bank will be required to report that deposit to the IRS on the 8300 form.