Ans is 3.
3 is the minimum stack size required. This is how..
First you will push 2 ... Stack size becomes 1
Then you will push 5 ... Stack size becomes 2
On seeing x, you pop 2 numbers from stack and push back the result... Stack size becomes 1
Then push 6.. stack size becomes 2
Then on +, 2 pops and then 1 push.. stack size is 1 now
Then on on seeing 4,2 they are pushed onto stack.. stack size becomes 3
Then eventually as expression is further evaluated stack size dwindles down to 0.
So to evaluate expression, stack size should be atleast 3.