STACKING
How much poison (psn) items, runes, and charms can you stack? Is there a max to the amount of psn you can use?
I'm not sure you realize this but you just asked one of the most complex questions in whole D2R.
Short answer : no, there is no limit on poison items you can stack. In theory, you can go wild and throw on everything you can find or buy that adds psn damage but in reality it is a very bad idea.
Poison Damage is measured in Bit Rate and Poison Length (in Frames). The values you see on items and the Character Screen are completely inaccurate (as is with a lot of things shown on the char screen)
The way psn is calculated in the game is very complex, influenced by the sources of psn damage (which may very well lower your damage if you combine the wrong ones) and is influenced by other sources of damage (physical/elemental) you, your merc or your hirelings do and even in such a way the other damage might negate psn bits.
To really understand the mechanics you better read up some nerdy poison damage guide (Maxroll's MacroBioboi is a good place to start = https://maxroll.gg/d2/resources/poison-damage ) and watch a few YT tutorials from established D2R content creators.
Pro Tip : find a way to cast Venom and it will be a lot easier to manage and calculate your psn damage, while your damage will drastically increase because it makes all your psn damage sources as coming from 1 single source.
Thank you for your insightful answer and the leads to further discoveries. I appreciate the time and thoughtfulness you put into your answer. This is the way all responses should be submitted.