about 3 years ago - Shurenai - Direct link
Vertical support is infinite when there is an unbroken line of blocks from bedrock to the block being supported- A central pillar(even 1x1) can thereby go all the way to the build height which is...pretty high, imo. It doesn't matter what material most of the pillar is either; even wood frames can. However, to build horizontally off the pillar the material that the floor/ceiling is connected to does matter; so it's best to use concrete or reinforced concrete at any joints.

Any openings/rooms along the way up, be it a mineshaft you made or a room you made will compromise structural integrity, so just make sure to add adequate pillars to support whatever floor you want to add and you'll be fine. And if there's a risk of zombie attack on a lower floor that they can gain access to, make sure to make it extra thick or out of a more durable material.

Remember: Unbroken line from bedrock; Any open space in the line makes the vertical support finite from that spot upward.

This here is a very comprehensive tutorial on how structural mechanics work. It's from A15, so some of the mass/support values listed are a little out of date; Always refer to the values in game to be sure; But the actual mechanics of how it works has not changed since A15, so the tutorial is still amazingly helpful.
https://www.youtube.com/watch?v=7hq_HxCqgyU