You mean, a loadout to summon the maximum number of things like the Queen Spider and Frost Hydra? You just, well, collect the items that spawn the sentries, and then summon them. I don't think I've actually attempted to summon two different sentries at the same time, but it does appear the queen spider and frost hydra can both be out, based on a picture and comments on the queen spider's page on the official wiki; this leads me to think you can spawn every unique sentry at the same time.
Keep in mind, though, that you get only one of any given sentry out at a time, period. Trying to summon a second hydra, for example, just spawns a new one at the targeted spot, and removes the old one entirely.