This is a really good question. I’m glad that you brought this up.
First of all, yes, the saints are all in heaven, and they remain limited as a creature. So no, they cannot be in multiple places, or can they hear all our prayers individually per se.
It remains a fact, however, that since the earliest times of the Church, beginning with the earliest liturgies, the Church has always prayed with and through her saints. They are our friends, yes, but also many favours and miracles were received through their intercession.
So how does it work?
Here is the part that the Church did not clearly define. I believe first of all that they know of our needs through God. In heaven they see God face to face, and so can see all that God can reveal to them. God does not reveal our inner thoughts and intentions, but all the things we articulate and make known can be made known to the saints.
Secondly, I believe they don’t need to spend a lot of time on each intention for them to intercede effectively for us. It is kind of like us too. Many of us have many people we want to pray for. We may even keep a list, and before long the list gets very long, and we don’t have time to pray for each one for long, or at all. Some of us start just praying for the whole list, knowing that God knows every single one of them. I believe the same may be happening with the saints. And God is pleased by their intercession not because they can spend time on each item, but because they love to help us.
And besides, I also believe that time works differently in heaven. The saints in heaven don’t have bodies, and as such they cannot change or do anything on their own. Theologians propose that the fact that they are alive and active in heaven right now is because God directly animates them in ways beyond their own nature. In other words, their activities are made possible because they participate in God’s life and in God’s time. But God is outside of time; so they must also have some access to the timelessness of God. And if that is the case, then they could read all our intentions a thousand times over before a second passes.
But as I said elsewhere, the truth is probably a lot more wonderful, and we will sure know all about it when we get there. So let’s make all our efforts to get there!