Personally I have (with a NAC-252) …
2XCD players (they sound different)
FM tuner
Record deck (actually output to StageLine)
Headphone amp (output to HeadLine)
Streamer (NDX)
That adds up to 6 and I can’t connect either my cassette deck or open-reel tape recorder without changing connections.
I don’t know why hi-fi companies don’t include lots of inputs. I assume the cost of adding a few more would be quite low.
I notice the new naim pre-amps/streamers have very few inputs.
I suppose the vast majority of people only have maybe 3 sources or fewer, often just one. Decades ago when I had TT, CD, R2R x2 and cassette, plus occasionally others, I made myself a multiple input switch box, which also routed the preamp tape out to multiple sockets.
I think 3 min. After that icing
It’s funny because so many people whined about all the “wasted” input on their 282 or 252 etc. But the thing is, once you have done the R&D on signal path for your product, adding additional inputs adds very little to the production cost of the unit, even a 552.
I’d agreed that three inputs (one of which must incorporate record out) is really the minimum. It is rare to see more than six though. Personally, on a full width preamp, six is a good number. On a shoebox amp, three or four. As with many things in life, too many is better than too few.
Just two for phono-satge inputs. That’s all that is necessary. 
2.5. Phono & Digital + Roon
For me the 222 has more than enough, a range of digital inputs and the DIN input for an NVC TT plus phono plugs as a spare input and of course the inbuilt phono stage.
For some others, I think around 10 analogue inputs would be ideal. And I know from many threads, the lack of Tape in/outs is an issue.
For Naim, I suspect it is a headache because whatever they do, some will complain.
Undoubtedly true!
For me the NSC222 has one too few X)
I run an external phono stage, so one DIN required
I also run a Naim CD player, so another DIN required
The NSC ran out already.
I get I could change ICs and run both sources, but I already invested in DIN cables for both, and don’t want to run RCA for one of them.
It’s a minor issue, but definitely puts me off considering the NSC as a future upgrade path. I’m probably not in the NSC league anyway, but it’s a bit surprising there isn’t a little bit more flexibility on its inputs
One. I handle the input switching on the DAC.
Cost is not the reason, if you try the Lejonklou pre the single-input mode sounds slightly better.
I agree ,all Naim Preamps I’ve had sounds better with only one source connected.
One more than you need… 
I don’t think I have ever used all the inputs on any of my Naim pre-amps…!!
NAC42 - 3 - used 1 (phono)
NAC72 - 5 - used 3 (phono, CD, tape)
NAC102 - 6 - used 3 (phono, CD, tape)
NAC82 - 6 - use 2 (phono, CD)
Same here.
It should have… enough. Failing that, I generally find that a pre-amp with too many inputs is usually far preferable to one with too few.
Like a Rolls Royce. 
For commercial reasons it must be so, of course.
Unless you solve it like Lejonklou where there are four switchable inputs. And one extra connector on the back that simply bypass the switch block.
In NORMAL OPERATION, the four inputs IN1, IN2, IN3 and IN4 are selectable. In the SINGLE SOURCE ONLY position, the four inputs are switched off and only the Single Source input is active.
If you only have one source component, the optimal way to connect it is to yellow input with the switch in the down position. Any signal on this connector goes directly to the volume control and then to the outputs. It can never be switched off, only adjusted in volume.
A big turn off for the upper range preamps for me is the number of inputs. I’ve never needed many inputs. Streaming and before that CD are/were my primary use. Maybe a tuner 25 years ago when I could still listen to broadcast radio. A big reason the 72 appealed to me was its simplicity. And for me 72/HC/250 was a nice balance for SQ and simplicity. When I had the 52 and later the 252 it seemed like I was wasting a lot of functionality for better SQ though the latter wasn’t worth it for SQ for me.
For me, two are enough. I have two DACs connected to the amp, and that’s all I use.
I’m using four of the six inputs on my 52.
Turntable
CD player
Cassette deck
TV
Planning to get a R2R one day, and who knows, I might even get a dedicated streamer as well. That would be all six inputs used up. If I needed more then I guess a DAC would be required.
So in answer to the OP, the more the merrier. 