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I'm a level 18 wizard. The Creature into Creature option of true polymorph states:
If you turn a creature into another kind of creature, the new form can be any kind you choose whose challenge rating is equal to or less than the target’s (or its level, if the target doesn’t have a challenge rating).
So I cast it on my self, being 18th level, to polymorph into another creature whose CR is lower than mine, let's say an adult silver dragon with CR 16, will I be an adult silver dragon CR 16 or 18?
True polymorph states:
The target’s game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.
Challenge rating is one of these game statistics1, and so your statistics are replaced by the statistics of the new form. So if you use true polymorph to turn into an adult silver dragon, your challenge rating will be 16.
That said, I am having trouble coming up with a scenario where the distinction would matter all that much.
1 From the introduction to the Monster Manual: "A monster’s statistics, sometimes referred to as its stat block, provide the essential information that you need to run the monster."
Player characters don't have a CR
CR calculations are for non player character creatures. A level 18 wizard, or indeed a level 18 for any class or combination of classes, does not have a CR of 18.
Changing into a creature with a CR of 16 doesn't give you a CR of 16. It just means you currently have the form of a creature with a CR of 16. A creatures CR reflects how dangerous it is in combat and how hard it will be to kill it. Since you revert back into your original form after your polymorphed form runs out of HP you aren't really CR 16.
