“What if a million people in the world pray to Mary at the same time?”
When the saints judge the fallen angels what if they have to judge a million angels at one time? What of it? Anything Mary can do she can do because of God. Is God stronger than a million people? If yes - and clearly it is yes - then why would that number pose a problem for His servants in Heaven if it is God’s will?
“Can she hear all of the at once?”
Yes - through the power of God. What’s impossible through the power of God?
“Are you saying she is omniscient or omnipresent like God?”
No, but she knows everything she needs to know to fulfill her role - again through the power of God. Jesus didn’t know everything in His earthly mission (Mark 13:32) and He is God! So if Jesus didn’t know everything - and He’s God - why would you assume Mary has to be omniscient or omnipresent to serve God?