I think it will be very difficult to find a confirmed count of all possible words for this. It seems very open-ended. Perhaps it should be switched to a contest to find the longest?
Everything below this is copied from @Kevin from a very similar questionvery similar question
I'm using the circa 2013 Words With Friends dictionary.
If you can only add to the right end of the word, 8-letter words are possible:
a
ba
bar
barb
barbe
barbel
barbell
barbells
a
ma
max
maxi
maxim
maxima
maximal
maximals
a
pa
pas
past
paste
paster
pastern
pasterns
If you can add to either end, 9-letter words are possible:
I
id
aid
aide
aider
aiders
raiders
braiders
abraiders
a
la
lap
laps
lapse
elapse
relapse
relapser
relapsers
I
in
pin
ping
aping
raping
craping
scraping
scrapings
a
at
eat
eath
heath
sheath
sheathe
sheather
sheathers
I
is
ais
rais
raise
raiser
raisers
praisers
upraisers
If you can add at any point, 11-letter words are possible:
I
pi
pig
ping
oping
coping
comping
compting
competing
completing
complecting
