35 cards in all.
Step-by-step
5 stacks with 7 in each stack means
5 groups of 7
This is illustrated as:
Group 1: 7 cards
Group 2: 7 cards
Group 3: 7 cards
Group 4: 7 cards
Group 5: 7 cards
Basically, 5 × 7
5 × 7 = 35
