Seven.
nor
6 letter words starting and ending with N:napkinnationneuronnotion
Put any vowel between, and you get a word: nae, nee, nie, noe, nue, nye.
lesion legion lichen lesson loosen listen lipton london
Overnight sojourn
nonagon
Niagara
Nesbia
nor
Pan, Pain, Pin
nervous
Narc.
marathon melanin
join
vision
One word that starts with "s" and ends with "n" is "salmon."
natio