As many as your computer can handle.
I don't think that is the correct answer unless environmental add-ons aren't loaded as datablocks when you start the server and are instead loaded when you first use them on a server. I know there is a default max datablock limit, I think it's around 40,000 datablocks per server. So does each texture count as 1 datablock or does each environmental skybox set count as 1 datablock? Let's say you only have default add-ons on, how many skyboxes or textures can you load under the max datablock limit after the default add-ons load? If the environmental add-ons aren't loaded as datablocks until you first click on them in the environmental GUI, how much can 1 server or 1 client keep in their memory until the server or client runs out of memory when only default add-ons are loaded in their RAM allocation?
On talking about the max datablock limit, is there any way to raise that limit or is it a forced limit you can't change?