Start off with a number of layers (6)
I believe the answer is:
strata
'number of layers' is the definition.
'strata' can be an answer for 'layers' (stratum is a kind of layer). I am unsure of the 'number of' bit.
'start off with a' is the wordplay.
'off with' indicates an anagram.
'start' with its letters mixed up with 'a' gives 'STRATA'.
Can you help me to learn more?
(Other definitions for strata that I've seen before include "Levels, grades or social classes" , "Geological levels" , "bands" , "Layers as of rock in the earth's crust" , "Layers into which rocks are divided" .)