Fine, change the term to "relationship expert". If you're good at maintaining a healthy romantic relationship, you'll have two, three, but optimally even one in a lifetime. That doesn't make you an expert, it still is the norm.

If instead you had 30 relationships, whilst for sure you had a lot of experience, I wouldn't really trust your advise as an "expert".

Sure. I was mainly concerned with the idea that most people are dating for marriage which I don't think is the case, at least under age 30.

But I think it's possible to be a relationship expert even with one or two real relationships. Two sets of people that have been married to only one person are not necessarily equally happy or fulfilled. A person could also make relationships their personal interest and spend a lot of time thinking about the topic on top of their lived experience which might make them more capable of giving advice.

I think it's wrong to say anyone giving advice isn't worth listening to.

The problem is that if you want to get married, you should take advice from someone who only ever had to go on one first date, not someone who keeps going on them.