It depends on the individual as well as the synths and of course the goal.
If you have a large setup it doesn’t mean that you necessarily need to use all of the gear all the time, personally I don’t like owning too much of anything, but I also don’t like not being able to make certain sounds if I want to, for example a 0-Coast is never going to make a resonant bass squelch on its own, a DX-7 isn’t going to sound like a Juno-6 etc. It can be fun trying to squeeze a non typical sound out of something that isn’t associated with that sound, but it can also be unsatisfying sometimes.
I like to have a certain palette of sounds at my disposal, and a certain amount of possibilities and also inter-connectivity, in an ideal world though I’d just have a computer and some decent controllers and a nice audio interface, then any single portable sketchpad type device like a OP-Z or Electribe or Deluge for capturing ideas.
But since I don’t have the patience for using a computer to make music, then I have to do it the hard(ware) way. I simply cannot stand waiting for a computer to boot up, keeping everything up to date, security patches, driver issues, having to wait around while the computer does something unrelated to what I want to do, software updates, new versions, etc. If someone ever came up with a fast music centric operating system on dedicated music computer hardware then I might be interested and get rid of most of my hardware, but in the meantime microsoft and apple already owe me hundreds of hours in wasted time so I’ll never try to use them again for music.
So, for me the answer would be a couple of polysynths, a couple of monosynths, a couple of drum machines, a couple of groove boxes, a couple of samplers, a few fx, a sufficient mixer, a modular.
That would be plenty, but even so I am under no illusions that I would actually NEED all that to make music that I would enjoy, any half decent groovebox could fulfil that role.