If there were millions, it allowed for some to survive by chance, etc, thus making the rare objects.

Perhaps they required a license, or permission from the Hall to be used. Sort of like fire arms regulations. Angreal probably were generally used for work related reasons, and the run of the mill channeler would have no reason to need one otherwise.

That the Forsaken were not sealed with any is indicitive that they were either a) not accessible to the general channeling population. b) the Light reacted quick enough and scooped up every available angreal and sa'angreal for the war effort. c) taking an angreal to Shayol Ghul might not have been a bright idea given the Dark One's control issues.

Most likely a combination of factors came into play, but I doubt millions existed. I think angreal were linked to jobs, and research facilities, and they had to be signed out and returned at the end of a shift sort of thing. That when the War of Power began, the Light grabbed up what it could, or smuggled out what it could to deny the Shadow tools.

Undoubtedly some Forsaken may, or may not have had them. If they did, they may have not brought them to Shayol Ghul for fear of the Dark One's displeasure, or a mixture of arrogance regarding their peers.