Not less than one, at least to begin with - later on in the game you can reduce the number using visbits.
Seriously, I'm pretty sure that one day someone really clever is going to come up with a TC which only has one system and no Spobs.
There was a very good episode of Voyager where the ship is trapped inside a nebula or area of space which has no planets and which it is (almost) impossible to escape from. Other ships trapped there have been raiding each other for stuff to live off and fight with, but Janeway hatches a plot to form a collaborative which enables them to escape.
Back in the glory days of EV, someone came up with the idea of a plugin (the term 'TC' had not been coined) which began with just Earth's system and then expanded as galactic travel was discovered. I would personally love to play a TC that grew in that way.
There was a flood of enormous game expanders for EV which added hundreds of systems. I have to say I never really got on with them I didn't want to explore loads of fairly similar systems, I wanted to find one system that really captivated me.
Ultimately for Nova you have to make as many systems as there are in Nova, otherwise Nova systems will start appearing randomly in your plugin. Most people seem to opt to make placeholder or null systems which then don't appear in the game.
Emotionally, I find the proportion and distribution of the systems makes an enormous difference to a plugin's ambience. Nova is quite sparse in some parts, and you spend a lot of time exploring empty systems or systems where you aren't (initially) allowed to land. EV was much more generous in this respect there weren't any really long chains of uninhabited systems except on the rim. EVO was between the two in places you could go a long way and find nothing, or at least nothing you were allowed to land in. I'm pretty certain that with the proper forethought you could layer a plugin into different ambiences where you made the player feel lonely, cosy, warm, cold, under threat, safe, hurried or languid just by sorting out the proportions.
Re your original question how long is a piece of string? Most plugins I've ever played really only had 25 interesting systems plus lots of filler worlds. How many worlds can you make which are distinctive from each other? If you can keep making the magic happen for yourself, then there's no reason to stop creating a new world every session you write, as a way of preparing yourself for mission writing.
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