A little more than a decade ago, I was speaking with an older gentleman of a relatively liberal political persuasion, and the topic of Y2K came up. (Remember that?) His response to me was, "I'm not worried at all about Y2K; I'm much more worried about Y6B." No, he wasn't just being clever about letter and number combination acronyms that begin with "Y," he was referring to the immanent reality (this was in 1999) that the world would be reaching an overall population of six billion. We are more than a decade on in time from then, and world population has surpassed that number by another billion, as of estimates for March 12, 2012. I remember thinking in the late 1980s, when we were studying China in middle school, that a billion people in one country was a lot, and that the world population being five billion was something that was inconceivably huge . . . and in thirty years, we've got almost half again as many people living in the world.
Rather than using this introduction to talk about the problems of population growth, the drain on the earth's resources that this creates, the necessity for taking birth control and family planning measures very seriously in a responsible approach to this matter, and any number of other expectable topics, I'd like instead to focus on a dimension that I'm sure many people—including many Pagans and polytheists—have likely never considered.
The two most populous countries in the world today are India and China. Though both are large, China is much larger, being one of the largest countries in physical geography as well as in population. Both have cultures whose histories extend multiple thousands of years into the past. Both have had varying successes as ascendant empires, or as oppressed or client-status colonized and imperialized nations as well. And, both are religiously pluralistic (although China has been less so for much of the last century), with at least three major religions holding sway in each for the majority of the last half-millennium: Hinduism, Jainism, Sikhism, and Islam in India; Buddhism, Taoism, and Confucianism in China. Of course, there are also a proliferation of minority religions, and a multitude of individual sects and theological movements within each of these major religions in each place. With histories and cultures that span the combinations of so many different population groups, and which cover such vast and different geographic features, it should come as no surprise that the animistic and polytheistic elements in each culture are extremely varied, and have been open to syncretistic influence for a great deal of their histories.
And yet, forms of Buddhism in China tend to de-emphasize plurality and focus instead on unity, and the major theological movements in Hinduism likewise tend toward monism in the modern period rather than polytheism. Individual deities, ancestors, and land spirits are subordinate to the oneness of Buddha-nature; and innumerable local deities have become epithets and by-names of the three major deities of modern Hindusim: Shiva, Vishnu, and Devi.
Turning to another populous and diverse Asian country, though, the results have been remarkably different: Japan. While Buddhism did make major inroads into the country around 1400 years ago, Shinto still thrives to this day, and many Buddhist bodhisattvas—and for that matter, Christian saints!—simply found themselves transformed into Shinto kami with relatively little difficulty. The vast national empires of India and China seemed to demand unity, at least as far as official stances are concerned; but Japan, which engaged in just as much (if not more!) complete cultural isolation at various periods in its history over the last millennium, instead did not enforce theologies of unity in its religious practice, no matter how unified and peaceful it attempted to keep itself in social and political terms, but instead allowed for a continued flowering of diversity, even though some kami who may have been separate beings originally did combine and create composite forms.
Because polytheism, therefore, tends more toward an acknowledgment of multiplicity and diversity, it is also equally well-equipped to deal with the introduction of new deities, whether these arise from religiously internal causes, or from contact with extra-religious forces (including other religions).A great deal depends upon the approach to animism involved, of course: going into a new land will always entail coming into contact with new land-spirits in the features of that new place. But, since territorial expansion is probably at an end for Japan, India, and China at this stage, the only real possibility for likely theomorphic population expansion in this regard is if Shinto shrines are set up in new locations around the world.
As polytheists of European extraction, however, a great deal of modern Paganism and other forms of practical polytheism face entirely different challenges. Again, whether animism is a significant part of one's practices will definitely influence the outcomes of this question. But, apart from that important consideration, how else is this question dealt with in practice, if at all?