Maximum depth of a B+ tree
Since you're not sure where you read it, is it possible you are misremembering slightly what the result was?

In a B+ tree, we require that every node has between $n/2$ and $n$ children.  In other words, every node is required to have at least $\lceil n/2 \rceil$ children.  This means that any tree of depth $d$ where all leaves are at depth $d$ must have at least $\lceil n/2 \rceil^d$ leaf nodes.  Thus, any such tree must have depth at most $\lceil \log_{\lceil n/2 \rceil} L \rceil$, where $L$ is the number of leaf nodes.  Perhaps this is what you read.

Note that the number of key value is related  to the number of leaf nodes: $\lceil b/2 \rceil -1 \le L/K \le b-1$.  This would let you get a similar result in terms of $K$, for a full tree with all leaves at the same level.

Detail: Here I have ignored that the root is usually allowed to have as few as 2 children.  This has only a small effect on the answer.