In God all things are possible. Perhaps it would help to ask what is a prayer...is it primarily a grammatical construct of words/thoughts expressing a particular desire to someone able to answer? Or do words merely clothe the prayer...movement of the heart and spirit towards one who can help? We are taught that the elders and saints often prayed beyond thought beyond words...face to face and heart to heart with God...simple total presence before God...and presumably vis versa. Consider the woman with the issue of blood what articulate prayer did she make...none, she merely reached out to touch the hem of Christ's garment and was healed.
If the saints are in Christ, then they too touch and communicate His glory, His power, His grace. Think of a sopping wet sponge touched by any number of dry paper towels. What is required for the towels to become wet...only to touch the sponge. The act of touching instantly communicates the water whether there is one paper towel or ten thousand.
The saints don't have to mentally cobble together/process one by one the thousands, or even millions of requests made to them. It is enough that they stand in the presence of Christ our God full of His love and mercy for us, full of readiness to help, present and dripping with grace...and with our prayers we touch them...our dry empty heart and need makes contact with their abundance in Christ. Their love, their prayer is not restrained by temporal concerns or limitations. It is simple, whole and wholy in Christ.
Besides, one might as well ask how can a Serbian or Egyptian saint from the fifth century understand a prayer made to them in modern English? If the Spirit helps them in that...what are numbers?