PFSRD: Pack Lord Druid Archetype, Pack Bond class feature:
The share spells animal companion ability only applies to one animal companion at a time—the pack lord cannot use it to cast a one-target spell and have it affect all of her animal companions.
I've occasionally seen people read this and go "oh, only one of his companions can have share spells at a time." Reading more closely and paying attention to the example, it's actually "oh, each time he casts a single-target spell, he can only share it with one companion that casting." That means...
Drd(Pack Lord)19/Wiz1 supplies 19 animal companions and one familiar, Extra Familiar grants a 21st. If you can find enough creatures (note that the spell-sharing collar doesn't grant share spells, only allows a creature to chain-benefit from it) that qualify, you can add 63 more to that with 84 of the collars.
I find it likely you'll run out of spells to transform them long before you run out of pets. I ~think~, if I'm reading that collar right (I might not be, it's late), you can instead use 20 of the collars to bind your pets together in sets of four, and allow a single shapechange or primal form spell to affect four of them at a cast.
Regardless, I think you're looking at a minimum of nineteen animal companions and two familiars before counting magic items using the Pack Lord Druid Archetype.
Optimization note: Honestly, I'd just go Drd(Pack Lord) 20 and take twenty animal companions instead of bothering with the familiars, so I could change them all in five shapechanges (if you have the massive Wisdom needed for that one additional level 9 spell per day, or a pearl of power IX).
