Interesting you should ask. Last night I was re-reading a book I've had for
some time Porsche: Power, Performance and Perfection. It's a photo/text book
by Susann Miller, ca 1992, with a LOT of history.

The various chapter numbers in the book might help a bit:

356 1948/1965
901 1963  (became the 911 because of a Peugeot numbering conflict)(*)
912 1965
914 1969  (to fill the price gap below the 911--HA!)
        (My '69 912 cost ~$6,000, the 1969 914 sold for ~$3,500)
916 1972  (914 with 911 E engine--<20 produced)
912E 1976  (limited edition to fill the sales gap between the 914->924)
924 1977 (4-cylinder, front engine, water cooled.
        (My 912 would run rings around it.)
928 1977 (6-cylinder, water cooled.)
944 1977 (6-cylinder, water cooled highway cruser.)
930 1976 (later renamed the 911 turbo. Not US because of emission standards)
(*)911 1963 (various models: (none), L, T, E, S, SC, Carrera, Turbo, etc.)
(911SC: special edition Weissach coupe, doric-beige leather interiors and
    contrasting burgandy piping on the seats as well as burgendy carpets
    with "911 SC" burnished into the center.)
959 1980 (matched performance of race car for streets)
968 1992 (Replaced the 944 coupe--six speed)

The rest of the book is on racing: 917, etc.

The intro is:
======
"It was decided early on that each project in the Porsche design offices would
be assigned a job number in sequence.  The job numbers would later become the
model numbers of Porsche automobiles.  The numbering started in 1930 with
design number 7, the Wanderer car chassis.  The engineers wanted the
contracting company to think that the new Porsche Company had past experience
in design, hence the number 7 instead of 1.  The Auto Union Grand Prix car was
number 22.  The Daimler-Benz land speed record car was number 80.  The 11 hp
diesel tractor was number 323.

"From the first 7 through the latest 968, the numbers continue to increase and
get better with age.
============

UKCARS.COM has a pretty good history. When the home page comes up, click
on "contents", then scroll down to "Marque History", then select Porsche.

Briefly, the 993 was the last variant of the air-cooled 911 family. The
996 was the first variant of the liquid-cooled 911 family; the 997 is
the second (a 2005 model year introduction). The 986 was the first
generaion "Boxster", the 987 is the second..........