There are 8 finals that have /-a-/:
Cantonese /-a-/ is like the Putonghua [e] in ‘的’ [de], and like the English ‘u’ in ‘gut’.
Note that Cantonese /-a-/ does not appear in syllables on its own, but rather together with a coda.
For beginners, the 2 final sets of /-aa- -a-/ are not easily distinguishable. For more, see ‘Cantonese Characteristics – Vowels /-aa-/ & /-a-/’.