As @luislast said, there is a difference between card (debit or credit) payments and direct pay. For example, for 1040 form/taxes due, the limit is 2 card payments per year, but 2 direct-pay payments per day.
For card payments, look at https://www.irs.gov/payments/frequ[product key removed]-by-type-of-tax-payment.
For direct pay, look at https://www.irs.gov/payments/direct-pay-help, expand question 6, "How often and how much can I pay".
I am pretty sure (99.99%) that one can combine these methods, i.e. send 2 card payments, and a few more direct-pay payments if needed.
