What is the maximum number of nodes in B-tree (of order 5) of height 2 (root at height 0) ?

In a B-tree of order $m = 5$, each internal node can have at most $5$ children. With the root at height (level) $0$, level $0$ has at most $1$ node, level $1$ has at most $5$ nodes, and level $2$ has at most $5^2 = 25$ nodes, giving a maximum of $1 + 5 + 25 = 31$ nodes in total.