Sorry to see that this is your grade 10 textbook. This also means you have no choice but to use this book for passing public exams. Please don't use L or NA as a symbol.
If I have one atom of oxygen , it has exactly L value.
No. Atoms are discrete objects i.e., countable. If you have 1 atom of oxygen, it means 1 atom of oxygen. If you said 1 mole of oxygen atoms, then it is 6.022x10$^{23}$ atoms of oxygen.
Your L comes from Loschmidt number or constant. From Science Progress in the Twentieth Century (1919-1933) , 1933, Vol. 27, pp. 634-649
Loschmidt 's number, N, is defined as the number of atoms in gram-atom
or the number of molecules in a gram-molecule.More than eighty
different experimental determinations of this number have been made
[i], and as it is a basic atomic constant its most probable value is
of great importance in atomic physics. It is, therefore, the purpose
of this article to outline the main methods by which Loschmidťs number
has been evaluated, and to give some indication of current opinions of
its most probable value
Footnote
1 This number is frequently referred to as " Avogadro's Number/' the
term " Loschmidťs Number " being then reserved for the number of mole-
cules in a cubic centimetre of a gas under standard conditions.
Unfortunately, these designations are often interchanged. Avogadro's
important hypothesis on the identity of the numbers of molecules in
equal volumes of different gases at the same pressure and temperature
was formulated in 1811, and is appropriately associated with his name
; but Avogadro made no quantitative estimate of either of the
above-mentioned constants. The first actual estimate of the number of
molecules in one cubic centimetre of a gas under standard conditions
was made in 1865 by Loschmidt, and from this the number of molecules
(atoms) in a gram molecule (atom) was later evaluated. From the
quantitative view-point it thus seems preferable to speak of "
Loschmidťs number per gram-molecule (atom)/' and of " Loschmidťs
number per cubic centimetre," as is almost invariably done in the
German scientific literature. This terminology avoids ambiguity, and
has been adopted here.
You can see here your textbook is using the symbolism of 1930s or earlier.