We had good responses for this question.
Here is a selection.
The first is from Justin at Chatham Middle School, USA:
Sarah could have had a total of $7$ cards. Will could have had a total of $4$ cards.
There are a plethora of ways to sort the polygons on the cards.
Some ways to sort the cards would be:
Shape: Circle, Triangle, Square
Color: Red, Blue, Yellow
Number of Sides: Zero, Three, Four
Number of Angles: Zero, Three, Four
Total Degrees of All Angles: $180$, $360$
These ways are ways that some of the shapes are alike, and therefore can be characterized as such.
Daniel from Kings School in New Zealand wrote:
Sarah wants to collect the circles and Will wants to collect the red shapes so therefore the cards that they would both want would be the red circles.
Not including the red circles, Sarah would get $5$ cards with circles on them; and Will would get two cards with red shapes on them.
It is also possible to sort the shapes into groups of squares/quadrilaterals and triangles. You can further break it down into right angle triangles and equilateral triangles.
Also you can group them into polygons, constructible polygons, regular polygons, scalene triangles, big circles small circles.
There are also six cards exactly the same as another. The yellow equilateral triangles;
The two small, yellow circles;
And lastly, the blue circles.
Lastly, Matthew at Moonee Ponds Central School, Australia sent in the following:
How many cards could they have had each? The most equal amount of cards they could have had each is $4$ cards with one large blue circle card left out.
How many ways can you find to sort the cards? There are $4$ ways to sort the cards among Will and Sara.
Can you see any cards that are the same as other cards? There are three pairs of cards, one pair of small yellow circle cards, one pair of large yellow triangle cards and one pair of large blue circle cards.
Can you see any cards they would both want? There are $2$ cards they both want, one large red circle card and one small red circle card.
Thank you all very much. These were excellent suggestions for solutions. It's good to see that more and more of you are expressing your ideas so clearly in words. Well done and we look forward to hearing from you again soon.