If the total number of active orders in any one account exceeds 10,000, any new orders submitted in excess of 10,000 will be rejected.
In addition to the limit on total orders in an account, there is also a limitation on stacked orders for any one contract, on the same side of the market (either buy or sell). If the total number of stacked orders exceeds 15, new orders are rejected. For example, if you had placed 15 separate buy orders for IBM, your attempt to place a 16th buy order for IBM would be rejected.