The answer should be 10. In a B+ tree the data is stored in the last level i.e on the leaf node. So going by the question-
Thus for a complete b+tree each internal level should have order_internal children.
if order is k and tree is complete-
Level 1 has k-1 keys and k children
Level 2 has k*(k-1) keys and k*k children if its internal node
Level 3 has k*k*(k-1) keys and k*k*k children if its internal node
and so on
Clearly here we have two levels of internal node whihc means k*k children i.e 3*3=9 leaf nodes
Also the order of leaf node is 2 so the number of total keys at leaf nodes=
No of leafnodes* order_leaf=9*2=18
Clearly here only 8 keys are present in leaf nodes so we can have 10 more keys inserted.