Life is too short to regret what you didn't have. My life journey is mapped out by my decisions of what I did do not by my regrets of what I didn't do.
What works for one family may not work for another. Hearing someone really regretting having only one child is not going to help you decide anymore than it will reaffirm your decision to have or not have another.
You make that decision based on your life. Whatever you decide will be the right decision for you.
When faced with big situations (like this), My DH and I make a pro and con list for the for and against elements and it helps. We've done it for:
private v state schools staying one child family v having another staying in present house v moving somewhere bigger
If you're unsure, consider thinking again about all the pros and cons in 6mths time and see if anything has changed.