That’s a huge rate. As a register is something like a call attempt, you are generating or the order of 720,000 busy hour call attempt equivalents. A PSTN local switch might only be rated for 50,000.
Is it huge regarding Asterisk limitation or hardware limitation? Server load is ok.
So I should consider to explode the load between 3 or 4 different instance of Asterisk?
It’s more a case that it is something that I can’t imagine Asterisk was designed to do, so it would not be surprising if it didn’t handle it well.
If you are going to put that sort of load on it, you are going to have to do your own investigations as whether it can cope and how it fails.