There used to be a maximum number of points but they've removed that ceiling. You can earn as many points as you can earn.
As far as the IRS payment is concerned, you may want to look into how it gets processed. If it is considered to be a cash advance instead of a credit transaction, you do not earn points on it.
You do not earn points for cash advances, convenience check transactions, service fees, interest penalties or for payments made on account services like credit protection.