It really does depend on many factors, some being your and their hand size, the amount of lands in play plus the additional lands you will be able to deploy in time and the composition of your hand, in general
While we have more than enough actual slivers to choose from, there are a few changeling cards that might be considered: Mutavault, Shields of Velis Vel, Amoeboid Changeling, Blades of Velis Vel, Shapesharer, Mirror Entity, Taurean Mauler, Chameleon Colossus
If you control no other creatures at that time or if the greatest power among other creatures you control is 0 or less, X will be 0
Ossification First off, that word means (basically) to harden into something like bone