?
I imagine it would be a rarity
Especially f they all formed from a collapsing cloud, like ours did
?
Nope as there is only one Solar system the one we live in . Our star is called Sol.
Stellar systems with a single planet are possible but should be very rare.
az_lender
We have discovered about 3500 planets and more than 2600 planetary systems. Therefore, most of the planetary systems discovered so far have only ONE planet that we have discovered at this time. I would not be surprised if there were many planetary systems that actually have only one planet ... though these same systems might also have the sort of objects that we characterize as dwarf planets, asteroids, comets, etc.
The earth has only one moon, right? But of course we now have loads and loads of orbiting objects such as defunct weather satellites and abandoned launch-vehicle stages.
quantumclaustrophobe
There are quite a few solar systems where we've only *seen* one planet - but, that doesn't mean it's the only one... I would bet there are a few out there, but I'd also bet there aren't very many.
ignoramus
We don't have the faintest idea. There are billions of stars in the Milky Way, and billions of galaxies in the observable universe. According to the accepted view of the evolution of planetary systems, it is unlikely that any star would have only one planet. But not inconceivable that somewhere or other a "rogue" system may not, through some cosmic accident, somehow end up with a single planet. But we would never know about it.