The simple answer is that this is in the realm 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.

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.

The simple answer is that this is in the realm 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.

The simple answer is that this is in the realm 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.