I mean, there could be thousands of Orthodox Christians at this moment praying to, say, Saint Blessed Xenia of St. Petersburg, and there could probably be even more of Catholic Christians praying right now to, say, saint Patrick, let alone Mother Mary. Does that mean that all those saints have the All-knowing God's ability to hear all of those praying to them, distinguish their prayers one from another, and later adequately act on each one of them accordingly?
Time has no meaning in heaven, there is probably a more precise Thomistic answer for this question bit that should suffice. If time is meaningless then "at the same time" is meaningless. I'll get confused if I write any more about that.
We not only believe that the saints can hear and deliver our prayers to God. We believe all the angels and all the saints are present at every Mass.
The answer is no. Recall first of all that prayer is not necessarily audible words, and secondly, the means by which prayers 'rise like incense before the throne of God' is never entirely clear. If we take for instance the issue of God hearing prayer, there are multiple ways in which the prayer could be received. To take two, the first is that God knew before the foundation of the world what would be prayed, though he himself did not make it so. The second is that through the Spirit he fills all things, and thus knows the words that are going to be prayed before they are formed into words.
We who venerate and ask for intercession of the saints believe that they dwell in God, so while it is not impossible that they can hear and interpret multiple 'messages' at the same time this does not seem to be an issue raised nor a description used. The saints live in the power and energy of the Divinity; they simply do what is given for them to do. They hear us through God to begin with.
Yes, this is granted by God himself:
'Now when He had taken the scroll, the four living creatures and the twenty-four elders fell down before the Lamb, each having a harp, and golden bowls full of incense, which are the prayers of the saints.' (Rev5.8)
'Then another angel, having a golden censer, came and stood at the altar. He was given much incense, that he should offer it with the prayers of all the saints upon the golden altar which was before the throne.' (Rev 8.3)
'Tobit, when you and Sarah prayed to the Lord, I was the one who brought your prayers into his glorious presence.... I am Raphael, one of the seven angels who stand in the glorious presence of the Lord, ready to serve him.'(Tobit 12.12,15)
'So it was, that while he was serving as priest before God in the order of his division, according to the custom of the priesthood, his lot fell to burn incense when he went into the temple of the Lord. And the whole multitude of the people was praying outside at the hour of incense.' (Lk 1.8-9)
The practise of the jews standing outside the temple and having a prayer service known as the 'hour of incense' is a type for the intercession of the saints and communion of the saints. Those praying in the outside court of the temple are us, the earthly saints of the church militant. Our prayers are offered with much incense by the high priest who is inside the temple serving God near the altar and the holy of holies. The high priest is Symbolic of those holy people that have triumphantly crossed over into heaven interceeding on our bahalf and spoken of in Rev 5.8. They serve as priests with Christ(rev 5.10)