There is NO reference of "soul" in the actual Bhagavad Gita translations!
"... and these jiva cannot be destroyed or created"
Probably due to ISKCON & few other translations, the famous notion of "soul is neither created nor destroyed" came into the popular culture.
However, during the discourse, Shri Krishna has not used the word jiva/soul except verse BG 15.7; And that context is also different. See the accepted answer of the below post:
Is every Jeeva "eternal" like the ultimate God (nirguna Brahman) or "temporary" within cycle?
Here are the actual translations:
BG 2.17 - But know, 'That' to be indestructible by which all this is pervaded. None can bring about the destruction of this Immutable.
BG 2.20 - Never is 'That' born, and never does 'That' die; nor is it that having come to exist, that will again cease to be. 'That' is birth-less, eternal, undecaying, ancient; 'That' is not killed when the body is killed.
BG 2.21 - O Partha, those who know, 'That' as indestructible, eternal, birthless and undecaying, how and whom does that person kill, or whom does he cause to be killed!
BG 2.22 - As after rejecting worn out clothes a man takes up other new ones, likewise after rejecting worn out bodies the 'embodied one' unites with other new ones.
--- from Gambhirananda's translations
'That' is referring to Atma (आत्म not आत्मा).
If you see these verses from ISKCON translations & few other translations, then you may find lots of reference of "soul", which is incorrect.
If "soul = Atma", then it is Not quantifiable
In the beginning of Gita (say chapter-2), Krishna was still acting as a rational person. Hence he referred the "Atma" as "That". Later, he established himself with the supreme one ("Brahman" or "Atma") and hence started rightly referring "Atma" as "Me". Here the "Me" was not individual to "Krishna", but an all inclusive "Me" to the supreme self or "Atma".
But, due mistranslantions, people have a notion of "soul" -- which kind of 'attaches' with individual body.
Now when the correct interpretation of "Atma" is associated with incorrect translation of "soul", a confusing theory got established that, everyone has their own "Atma".
From Gita perspective, "Atma" is the supreme reality ("Nirguna Brahman"), which is in Oneness (not the quantity one). So if you think soul as Atma, then the soul of yours, mine, elephant, earth -- is no different.
If "soul = purusha/jiva", then the number could be very large, but not infinite
IMO the interpretation of "soul" as "purusha/jiva" seems fair (refer Gambhirananda, BG 13.22). A Purusha resides where the emotions like happiness, sadness are consumable.
BG 13.22 - Purusha (soul) being situated in Prakruti, experiences the [sattva, rajas, tamas] modes born of Prakruti. Contact with these modes, is the cause of its births in good and evil wombs.
That means, a microorganisms like cells or bacteria inside our body may have their own souls, a humans/animals may have their own souls, our family may have its own soul, a society may have its own soul, the mother earth may have its own soul and same goes for solar systems, galaxies etc.. The ultimate soul of universe (BrahmAnda) is known as "super soul" (i.e. "Saguna Brahman").
Wherever you see a 'system', there is a possibility of a soul for it.
The cycle of Brahma is finite, hence there is nothing in that which can be infinite. Since, the creation of infinite items will take infinite time, it will go forever and the cycle will never end. But cycle ends upon the arrival of night of Brahma (refer BG 8.17-8.19, 9.10). This implies that number of "soul" in this context can be a very large finite number, which is not worth counting.