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
nervous
Pan, Pain, Pin
Narc.
marathon melanin
vision
join
One word that starts with "s" and ends with "n" is "salmon."
natio