How can the Saints intercede for millions of simultaneous praying Christians at the same time?

I am not quite sure I understand your question. You said "how can a Saint intercede for the millions of Christians praying to them at the same time?". The picture represented in this page in Revelations was also seen in the tabernacle and temple setup by the Israelites during the days of Moses and the kings of Israel. The Levitical priests would offer up incense on censors in the tabernacle and in the temple. The incense when burned would represent a pleasant aroma that went up before the Lord. This incense was symbolic of prayer. Angels are not considered "saints". But angels are active in hearing the requests of believers and bringing them before the Lord which is symbolized by the picture in Revelations. Remember, there was a story in the book of Daniel where he fasted. And one of the Archangels was dispatched by God during the time Daniel starting fasting and prayer. The angel came in response to his fasting and prayer, because the Lord had heard and seen Daniel's fasting and praying. So, the picture in Revelations is symbolic and represents the prayers of believers being heard and pleasing unto the Lord.