The standard here, in more recent boats, is twins, often plus bowthruster, OR single plus thrusters both ends. And some twins, with thrusters both ends,sometimes with integrating Yacht Controller.
Problem is, I think, anecdotally, thrusters fail more engines,and if they do, an operator used to manouvering/docking with thrusters has to work without. I`ve seen it happen on our marina, and suspect it`s a greater problem than engine failure needing redundancy. And it`s likely to happen just when you don`t want. like if you need the thrusters for extended periods and they overheat or drain their batts. However that redundancy can be mighty helpful for manouvering.
And that`s why I prefer twins. But read up on the old threads, no lack of them.
The hairy chested Swinging Ds will tell you a single no thrusters is all you need, but I disagree, some help is no bad thing.