When Phyrexian Rebirth resolves all creatures are destroyed and then a token creature with put into play with power and toughness equal to the number of creatures that were destroyed is put into play. It is important to note that this may not equal the number of creatures that were in play when Phyrexian Rebirth started to resolve. Creatures that are not destroyed, either because the are indestructible (like Blightsteel Colossus) or have regenerated (like Thrun, the Last Troll is capable of) will not be counted when calculating the power and toughness of the Horror artifact token creature.