I love your profile picture, I just want you to know that.
Also, I would like to input my opinion. I believe that Kabuto is smarter than he is arrogant, and thus would make sure to prepare all of his edos as soon as he can to have them ready to fight. So from my perspective, he could, if he wished, summon all of them at once in any particular battle.
Also, since when we do things like this where we compare the strength of characters we take them at their most powerful, it would only be logical that we take Kabuto when he is at his most powerful, which is when he has all of his edos prepared and ready to summon at the clap of his hand.