What's the amount of blood passing through heart every second at Max Heart Rate?
· Heart Beat Rate


  The normal heart has a rate of 72 beats per minute, but in infants the
  rate may be as high as 120 beats, and in children about 90 beats, per
  minute.


Lets say the heart beats at an average pace of 70 bpm (beats per minute)
(~100.000 times a day)



· Cardiac Output

Volume of blood pumped by the heart in a minute.

CO = HR X SV (heart rate multiplied by the stroke volume)

Cardiac output is the volume of blood being pumped by the heart in a minute. It is equal to the heart rate multiplied by the stroke volume. So if there are 70 beats per minute, and 70 ml blood is ejected with each beat of the heart, the cardiac output is 4900 ml/minute. This value is typical for an average adult at rest, although cardiac output may reach up to 30 liters/ minute during extreme exercise. 

So for an average adult at rest the estimate is around 5 litres of blood per minute (~1.33 gallons)



· Cardiac Outout / second

There are 60 seconds in a minute so simply : 
divide amount pumped per minute by 60

0,083̅3̅  L of blood per second
(83,33 mL of blood per second)

But this is just an average with specific variables defined... so maybe you have to take into account other factors that may vary this value.



· Factors Affecting Cardiac Output


  Factors affecting cardiac output in a healthy but untrained individual
  could be:

Increase/ decreased in heart rate
Change of posture
Sympathetic nervous system activity
Parasympathetic nervous system activity can also affect cardiac output.

  
  Heart rate can vary by a factor of approximately 3, between 60 and 180
  beats per minute, whilst stroke volume can vary between 70 and 120 ml,
  a factor of only 1.5.


You can read more here &amp; here