The IRS doesn't really have a limit on the installment plans. You can add your current balance to your last year's balance and there will be just one installment agreement that will include both amounts. But if you already have one and would like to add another, you must contact the IRS as soon as possible to have it included because a new balance will automatically default your current agreement (It is one of the terms and conditions.)
You can call them at 1-800-829-7650 and expect long wait time...