1. Pick a blank character
The classic blank-name character: a Korean filler letter that shows as a wide empty space. Because it is a letter, not a space, apps that trim spaces usually keep it.
Blank names: Often worksBlank messages: Often worksNot a space (survives trimming)
2. How many
What you are copying
HF
As it really looks, between the brackets: [ㅤ]
Every app decides which characters it accepts. If one is removed or rejected, try another kind. Please don’t use blank names or messages to pretend to be someone else, to dodge a ban or to spam: most communities treat that as against their rules.