In all likelihood, this is a leak meant to gauge reaction. This way, if there is a strong backlash, the president can backtrack a bit and go for a smaller number; or if it's weak, it may go as planned.

As was mentioned above, this is politics and Obama is entering what is probably unknown terrain. This executive action may cut multiple ways. Republicans will go apeshit, that's for sure. What is not certain is who is going to get wounded by this.

The Republican reaction might be so extreme that it splinters the party and alienates mainstream public opinion and minorities. Or alternatively, public opinion might recoil at what it sees as lawlesness on the part of the President; this in in turn might make various Republican countermeasures from court challenges to government shutdowns to impeachment politically feasible and popular and that's obviously something Obama would rather avoid.

To me the latter part is a real danger and that's why it seems to me that Obama will choose the most legally and politically defensible action he could take. This means that in all likelihood, the number would be less rather than more.

And I am saying that as someone who stands to gain by the executive action being more rather than less.