From this season's CL regulations:
18.08 No club may have more than 25 players on List A during the season. As a
minimum, eight places are reserved exclusively for “locally trained players”
and no club may have more than four “association-trained players” listed on
these eight places on List A. List A must specify at least eight players who
qualify as being “locally trained”, as well as whether they are “club-trained” or
“association-trained”. The possible combinations that enable clubs to comply
with the List A requirements are set out in Annex VIII.
18.09 A “locally trained player” is either a “club-trained player” or an “associationtrained
player”.
18.10 A “club-trained player” is a player who, between the age of 15 (or the start of
the season during which he turns 15) and 21 (or the end of the season during
which he turns 21), and irrespective of his nationality and age, has been
registered with his current club for a period, continuous or not, of three entire
seasons (i.e. a period starting with the first official match of the relevant
national championship and ending with the last official match of that relevant
national championship) or of 36 months.
18.11 An “association-trained player” is a player who, between the age of 15 (or the
start of the season during which the player turns 15) and 21 (or the end of
the season during which the player turns 21), and irrespective of his
nationality and age, has been registered with a club or with other clubs
affiliated to the same national association as that of his current club for a
period, continuous or not, of three entire seasons or of 36 months.
18.12 If a club has fewer than eight locally trained players in its squad, then the
maximum number of players on List A is reduced accordingly.
So the answer to the original question, how many Scottish born players do Celtic have to have, is none. As long as four of them have been with Celtic for three years as youngsters, and another four with any Scottish club as youngsters, the whole squad of 25 could be foreign born.