Very interesting notion - I've wondered about this too. Seeking some answer for us includes referring to the C's material, supplemented by lots of other sources. The C's have mentioned OP's among human-appearing entities, and they indicated that there are many on earth now. Bringers of the Dawn relates that many souls have reincarnated during this timeframe because of its universal significance, and even that some have come simply as observers. The writer of A Dweller on Two Planets related that between two incarnations he was "asleep" for 40,000 earth years, in a place similar to the C's description of 5D, to contemplate his evolution. When his reflection / assimilation was completed and he was prepared to incarnate again, he chose to do so. Could be that every soul, individuated, needs individual care, balanced with the needs and growth of its soul group and of an even larger group, a soul memory complex. In certain periods I'm sure there is correspondence among souls, when many contemporaneously choose to incarnate, perhaps for the purpose of doing some common work. This idea I know has been treated by the C's.
I don't necessarily see these sources as equivalent, but they sure are from disparate places! Why so many now? The C's also answered in response to Laura's question of how many were on the earth 309,000 years ago, aligned with the cycles, that that number was around 6 billion, similar to now, if I remember - though the exact reference isn't at my fingertips.