How many open spaces above my planter boxes should I have?

[mr.j.j.borreli]

I'm working on building an herb farm and I'm curious about how many open spaces need to be above a planter box with a seed in it.

Is it different for certain plants because I think I have seen moonglow seeds grow when there was only 1 free space above the planter box.

Hopefully someone can provide some insight here I would really appreciate it!

[VagueCryptid]

You only NEED one, however it might look better if you had 2 or 3. One is plenty for the efficiency route.

Tip: You can stand on planters; some people don't know this for whatever reason.
Tip 2: Get a staff of regrowth; Makes farming plants way easier.

[mr.j.j.borreli]

Thanks a ton!