I should think?

Ned rose and pulled her to her feet. “Damnation, how many years has it been?
And he gives us no more notice than this? How many in his party, did the message say?”
“I should think a hundred knights, at the least, with all their retainers, and half again as many
freeriders. Cersei and the children travel with them.”

Dear teachers, would you please tell me what "should" means here?

Thank you.