When a person dies, they aren't immediately reincarnated. They usually spend about 30-60 years in the spirit realm before they incarnate into their next lifetime- in the spirit world they heal, review their past lifetime, and then plot out their new one. Some very advanced souls only reincarnate every 200-300 years and only incarnate as teachers and spiritual leaders. So, when mediums connect with people who have died, they're connecting with the soul in the spirit realm before it reincarnates into another physical body. There's usually a large period of time to do so before the person incarnates again.
In very rare cases, a soul may immediately reincarnate after dying if the death was unplanned and sudden, such as dying in a war. These people will immediately "jump" into a fetus and be reborn into a new lifetime within 12 months of their death in the past life so that they can fulfill their life plan. These are the children who can remember their past lives and have vivid memories of past incarnations- I'm sure we've all heard of such children.
As for new souls being created, yes, that does happen. Souls tend to be created together in a group, and this group typically incarnates together as various different roles (lovers/spouses, parents, children, siblings, best friends, even coworkers, in some cases).
But, from what I hear, brand new souls don't incarnate into the "modern world"- they often incarnate into less advanced tribal societies where they can learn the very basics of being human and being part of a family.
These "baby souls" are typically born into regions without Internet access and are usually illiterate as well, from what I've read. This is to shield them from being exposed to too much complex information and contradicting perspectives that would hinder them in learning the very basics of functioning as a human in daily life.
Only strong souls incarnate into modern societies because the amount of contradicting information and the sheer amount of difficult moral choices we have to make on a regular basis throughout our lives would be too overwhelming and confusing for a brand new soul.
Most souls eventually want to stop reincarnating on earth, so after a certain amount of lives and after learning and experiencing all that they set out to experience in the physical world, they "graduate" from reincarnating and can stay in the spirit realms, sometimes serving as guides to other people in their soul group who are still incarnating or to spiritually inclined people. So, I would imagine new souls are created to make up for the souls who stop reincarnating.
This is assuming that reincarnation is real. If it's not, then after death we all fade into nothingness and that's the end of it. It's up to you to determine what you believe. I hope this answered your questions