I'm a parent of two, currently aged 6 and 3.
I agree with others that making a decision to have a second child should wait until you have had your first. It is impossible to truly understand parenthood until you try it.
Plus you never know what life will throw at you.
I went through a couple of periods in which i thought I wanted kids. Once was when I was going through puberty, so I believe that was hormonal/biological. The second was at around age 20 but it was more a fantasy of having a cute trophy than anything biological.
Despite not using contraception in most situations, I never fell pregnant. I assumed I was infertile, which worked fine for me and felt like fate. Later I learnt it was due to endometriosis.
When I met my second husband, I came up against the question of having kids. There was something different about this guy... It felt like fate that we met (we still feel this very strongly). He had always wanted kids. For the first time in my life I felt I really could see myself having one. After both having been burnt in our first marriages we decided to give our relationship a year and then we'd try for a baby. But fate had its way and I was pregnant two months after we met. Bang, like that. I was terrified.
I didn't know then that I am autistic. But I was obsessed. To try to cope with this huge change I had to research everything and find out everything... After the birth I faked happiness for so long, but I was resentful and frustrated at losing my independence, freedom, solitude...all those Aspie things I need to survive. I love my son, but...
While I was still in fake-perfect-mummy mode, and my hormones were still going crazy, I decided a second baby would be great. Get them all out of the way quickly, then move on to the next phase. But I miscarried, and we didn't feel like trying again for a long time. And in that time my hormones rebalanced and I had time to think. I drew up a table weighing up the pros and cons of having another baby. The table overwhelmingly leant towards the cons. I realised that if I had a hard time coping with my son it would be much harder with two. Something told me I was too selfish/self-interested to love a second child. So time passed.
A year and a half later I accidentally fell pregnant again. I was devastated, while my husband was ecstatic and refused to let me have an abortion... (This is a grossly simplified version of the story)... We fought about it and it was really tough, but somehow I eventually accepted that I would have to go through with it or risk our relationship. As the early weeks passed I knew instinctively that I was having a girl this time, and this changed things for me. I don't know why, it was a biological mother-daughter thing. I started to look forward to meeting her, which I hadn't done with my son at all. When the ultrasound showed it was indeed a girl, I was elated. I approached this pregnancy very differently and prepared myself psychologically.
I've tried to keep this short, and failed miserably, sorry! I have left out much that I feel is relevant, but this is getting ridiculously long!
For me, as an autist, two kids is incredibly hard going. Having kids has exacerbated my bad traits. My anger is always near the surface, and I suffer sensory overload often. I now keep noise reducing ear muffs in the kitchen for bad times. Our eldest we suspect is also Aspie. This brings extra challenges, and he and I are so alike yet we constantly clash. There is also my dissatisfaction at having my day dictated for me, despite loving routines. I feel like I'll never do anything meaningful with my life (although I am aware that having children is pretty meaningful, especially if they are bright and talented like my son). I don't cope well with those unexpected things that come with kids: wanting friends to come over at the drop of a hat, or emergencies, or kids not showing up on time, or sudden illnesses that require my constant presence and impinge on my personal time, etc. The social side of parenting is especially troublesome... My son wants to play with friends but I don't want to talk to anyone or their parents. I've never been a kid-friendly type!
But despite all the hard stuff that I'm struggling with, I also know that without these struggles I wouldn't have found out I'm autistic, because it was my search for answers to explain my constant anger with my kids that led me to it. Since then I've had an amazing awakening, finally understanding so much about myself.
I don't know if any if this will help you. But know that you are not alone in changing your mind. Remember than any choices you make now are not set in stone... Life never goes the way you plan it, as I well know with two unplanned kiddos!
I'm not sure how young you are (I'm soon to be 40), but my most conflicted time as far as do-I-don't-I-want-kids was from my mid teens to around age 22. After that I moved overseas and was too busy having a good time for a decade that the question never really came up (thankfully, for me). I'm a bit of a fatalist and so no matter what you feel now, nothing can predict what will happen when you're older.
Sorry, everyone, that this is so long.