I have only one ferret. Albino female named lumi. She is a little over a year now. She is our first ferret. At the time we could only afford one and since I am a stay at home mom attending to the ferrets daily needs was very doable.
She seemed quite happy to just play with and get attention from people. But mostly she enjoys exploring the house.
We are moving soon and talked about getting another ferret once we settle down in a new home. We talked about it because we decided we are very much ferret lovers and since we have done well with one we think two will be no problem.
So my reasons for only owning one ferret was because at the time we could only afford one.
I think with plenty of attention owning just one is perfectly acceptable.