The most common and most likely answer to this crossword clue is the 8 letter word PLANKING
I've seen this crossword clue in The Sunday Telegraph
P
L
A
N
K
I
N
G
More clues leading to the result PLANKING
Buddy explains!

The straight part
We know that "Number of boards" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case number of boards = planking.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
Crossword Answer definitions
PLANKING verb- To cover something with planking.
- To bake (fish, etc.) on a piece of cedar lumber.
- To lay down, as on a plank or table; to stake or pay cash.
- To harden, as hat bodies, by felting.
- To splice together the ends of slivers of wool, for subsequent drawing.
- To pose for a photograph while lying rigid, face down, arms at side, in an unusual place.
- A series of planks; a group of planks.
- The practice of lying face down with arms to the sides, in unusual public spaces, and taking photographs to record the act.