The simple answer is that this is in the realm of DM authority. How much change in mass the DM is willing to put up with is a DM decision.
Example 1:
If someone wants to true polymorph Saint Helen into a mountain, and DM would like a Mount Saint Helen to be based in that origin for the name, then maybe the DM gives the OK.
Example 2:
If a player wants to true polymorph Saint Helen into a mountain sized pile of pure platinum, DM just might roll eyes and not allow it.
A way to answer this question from a game mechanics perspective is:
What is the CR of a mountain?
This borrows from the "Creature to Creature" description on p. 283 of PHB, but I think this gives at least a guideline for bounding the problem.