In our case, our one and only DD is much loved, but definitely enough for us.
She has some high functioning SNs, which means she still goes to mainstream school but we have more issues at school than many others do. But we've had the time to focus on those, and seen huge improvements as a result.
DH and I both work FT - and both of us actually like to work as well as it being useful to pay the mortgage and put food on the table. But it would have been much harder for me to keep going as I have had I been dealing with a second or more DCs.
We also found that, as DD got older, we started to get a semblance of personal lives back again too. We found a babysitter for occasional nights out, and we've also been happy to organize her doing whatever she wanted (within reason) outside of school. So she's done lots of PTA organized extra-curricular activities in school after school has finished (at varying costs), and been involved in various sports and other activities in the evenings/weekends as well (gaelic games and swimming lessons for a number of years, Coder DoJo for computer coding for about 2 years and she learned guitar for a year as well, she still plays hockey (although we had to give up on an outside club due to timetabling clashes - but may get back to that), and she does (a lot of!) sailing and is a very active SeaScout as well). As well as surfing and lots of sailing during the summers, and going to other camps as well when she was younger like cookery, multisports, and gaelic games and hockey camps rather than just staying in the crèche/afterschool club summer childcare provision. There's no way we could have paid for all that, or juggled the logistics!, if we had 2 to cover.
DD has plenty of friends, at home, in school, in Scouts and in sailing in particular, so hasn't lost out from not having a sibling. And she has DCousins that she sees when we get to visit "down home" or overseas DSiblings of ours get to "come home" as well (home being where we grew up rather than our own homes as adults).
We love being a family of 3, and DD does as well. We've gone on lots of holidays we probably wouldn't if trying to facilitate younger DCs or bigger groups. And now, aged 12, DD is quite happy to be quite independent at times (walked home from school alone from early 5th class, due to bullying issues in school was coming home once afterschool activities finished in 6th class rather than using the afterschool club (childcare) and did her homework and made her snack before relaxing for a couple of hours - she got home around 3.30-4pm and we arrived home about 6), she gets the bus down to the local town herself and loves doing a few jobs for us (get a bottle of milk, post a letter, small jobs like that) and even the chance to meet DH for a "coffee" in between meetings on occasions (I'm based in city centre).
There will be loads who will say you should have more. There will be loads to say 1 is enough.
You can only decide what suits YOUR family unit. And each child is different as well, so while it may be the same a 2nd time around, it may well be a different experience too.
While part of our reason for staying as a single DC family relate to her SNs, a lot of it is not. And we're very happy that way, seeing how many of my friends and family members struggle more with multiple DCs and the guilt they feel over not being able to give enough attention to each of them, compromise over outings/activities/food choices etc because of more DC, and the logistical juggling between schools and childcare and guilt about work and days to cover illnesses and all sorts.