Antipope Alexander V reigned in years 1409-1410, but the real Pope Alexander V reigned from 1492-1503.
Antipope John XXIII reigned in years 1410-1415, and the real Pope John XXIII reigned from 1958-1963. The latter pope started the Second Vatican Council.
We don't count antipopes.
Let's look at the names of the Popes and antipopes of the time of Western Schism in Annuario pontificio (Rome, 1869)
. If I understood correctly Italian title, they correspond to the series of portraits of the Popes in Papal Basilica in Rome St. Paul Outside-The-Walls.
Indeed, we can see that later there were Popes with the same names as antipopes at Avignon:
Clement VII (1378-1394) had the same name as Pope Clement VII (1523-1534);
Benedict XIII (1394-1417) — as Pope Benedict XIII (1724-1730).
But the Pope who reigned in 1492-1503 is under the name Alexander VI, not Alexander V. Doesn't this mean that Alexander VI (1492-1503) considered Alexander V (1409-1410) as real Pope?
I took a look at that old Annuario Pontificio. It does sound that the numbers were confusing at that time. For example, there is an entry that states:
207. Giovanni XXII, o XXIII, o XXIV, Napolitano, Cossa, c. 1410, G. A. 5, g. 13.
The word "o" is Italian, meaning "or".
I do have an updated Annuario Pontificio, dated 2005 and printed at the Vatican City. Here are several lines from page 17*:
Gregorio XII, Veneziano, Angelo Correr, 30.XI, 19.XI.1406 - 4.VII.1415.
[Clemente VII, di Ginovra, Roberto dei conti del Genevois, 20.IX, 31.X.1378 - 16.IX.1394].
[Benedetto XIII, di Illueca (Aragona), Pietro Martinez de Luna, 28.IX, 11.X.1394 - 29.XI.1422 o 23.V.1423].
[Alessandro V, di Kare (Creta), Pietro Filargis, 26.VI, 7.VII.1409 - 3.V.1410].
[Giovanni XXIII, di Napoli, Baldassarre Cossa, 17, 25.V.1410 - 29.V.1415].
Martino V, di Genazzano, Oddone Colonna, 11, 21.XI.1417 - 20.II.1431.
Antipopes are enclosed within brackets and right-justified in the latest Annuario Pontificio.
That book also has a footnote for Alessandro VI, di Jativa (Valencia), Rodrigo de Borja, II, 26.VIII.1492 - 18.VIII.1503.
The footnote for "VI" states that:
Can be "V" since Pope Alexander V in the Pisa Council (years 1409-1410) was considered illegitimate.
There is a list of popes, along with anitpopes (enclosed within brackets):http://iteadjmj.com/docs/pontefici.pdf