How can I increase bifido bacterium strain density in kefir
The simplest way to increase the diversity of strains might be to start from a mixture of kefirs from the most diverse and independent sources you can find. There's no guarantee that putting two symbiotic communities together will improve either of them though, or produce a final more diverse community.

What are the constraints on density? I would imagine that density will increase until either nutrients are depleted, or pH or other toxicity factors reach a certain limit. You might find that some strains tolerate a higher/lower pH range than others, and also that careful pH adjustment would allow density to keep increasing, provided you supply enough nutrient.