No one gets any guarantees about anythiing.
You weren't guaranteed the right relationship, at the right time, to have the right child at the right age. So you are being proactive and adopting a child who needs you --which is great and very giving of you. But please don't mourn what you don't have and never were guaranteed, which is a second child.
Bear in mind: Adoptions do have problems, and you don't have the first child yet; I hate to say it but the fact you're high up on the adoption list doesn't guarantee you'll have a child tomorrow, or even soon; parents can change their minds about giving up kids for adoption at all. Will you then grieve that first child on top of grieving over a second child who isn't even on anyone's horizon? Focus on one child at a time.
You haven't tried parenthood with just one yet. Though right now you are convinced two is your magic number, you may find that one is right for you. Children are demanding, at all ages. You are the same age as me (my daughter is nearly 11); do think about this: If you adopt a baby right now, you will be 57 when your child is 10 years old; 65--retirement age--when your child is 18 and just starting college; and so on. Let's be very clear here: I am NOT saying you're too old to adopt; no one is too old to give their love and lifelong devotion to a child. But I am saying that especially with another adult in the picture, you should consider some tough things that no one likes to think about, like finances and your own later years. What were your plans for your own retirement; your finances for your child's education and other expenses; your energy level and ability to spend time with your child in those first 18 years, and so on? One child may be perfect for that, but two could tip you into the territory of having to work much longer than intended, or do without retiring, etc. It sounds selfish to discuss it, right? But doesn't talking about it now beat getting a few years down the road to find your new husband, and maybe yourself, thinking, two was one too many?
I would discuss all this with the gentleman in question, frankly and openly. If you feel you cannot discuss with him these seemingly "unfeeling" but very important things, like finances and your ages relative to a new child, you should think again about whether he's perfect for you; these are the brutal but realistic questions that parents have to face, and if you can't discuss them with him -- is he OK with being a 65-year-old dad of a new college kid? Does he buy in 100 percent that this is his child, 100 percent, as much as yours? and so on -- well, if you can't talk about that, it's no basis for a relationship.
By the way, only kids are not necessarily spoiled. They are not necessarily lonely. They are what their parents make of them, and if the parents spoil them or keep them on a short leash they'll have the problems you'd expect. I love having one child. It has enabled me to be MUCH more involved in her school by volunteering, something I see parents with two or more kids often can't do because they have a younger one to deal with so can't be at the older one's school. Having one child means having the time to help with scouting or other activities as you are needed. I know that parents of more than one do manage it but it is quite frankly easier with one. And kids do not "need" siblings for companionship or playmates or lifelong friends; I know plenty of adults whose siblings have nothing in common with them and who even dislike their siblings. Again-- there is no guarantee that providing a sibling for your child will provide a lifetime of warm memories and closeness with that sibling.