1: Out of the Mithril Mine, but before sleeping at Junon
2: After sleeping at Junon but before getting Shiva
3: After getting Mr Dolphin's whistle but before using it
4: After getting the buggy, but before Cosmo Canyon
5: After Bugenhagen asks you to form a party to go into the Gi Cave
6: After Cosmo Canyon events but before talking to Sierra
7: After talking to her but before the arrival of Rufus in Rocket Town
8: After getting the Tiny Bronco but before getting the Stone Key from Dio
9: After the night at the Gold Saucer but before entering the Temple of the Ancients
10: After you wake up in Gongaga but before using the Lunar Harp
11: After having used the harp but before sleeping in the Forgotten City
12: After sleeping but before end of disk 1
13: At the beginning of disk 2 but before you're taken to Junon
14 - 15 - 16 - 17 - 18: Impossible to attend those because Tifa is kept at Junon
real 14: When Tifa's leading but before she finds Cloud at Midhir
15: Impossible to attend, you're with Cloud in Midhir
real 15: Huge Materia
EDIT: just adding, when the folks at the Fort fight in your place, it's not like: you've skipped battle 7 ? We've fought it in your place!
nono
Each battle equals a determined number of points, if you've skipped for 70 points worth of battles, a battle occur while you're away (and for each 3.000Gils they have, one battle is considered won by them)
(Two of these 15 battles are worth more than 70 points, just by themselves).
So for example.. if you do battle 1/15, and do not battles 2/15, 3/15, 4/15, 5/15, 6/15, 7/15 (those 6 are worth 179pts), the folks of the Fort will have waged 2 battles during your absence, and when you get there for your 2nd personal battle you'll be proposed to wage battle 4/11... n°4 because: your 1st + 2 by them --> this one now is the 4th... and /11 because: 15 (total) - 6 (skipped) + 2 (by them) --> total of 11 remaining possible battles in the future.