Cylindropuntia californica var californica is a rare cactus that rarely produces viable seed. How many cuttings from individuals from a small population of approximately 30 would be needed to create a living conservation collection?
I also agree. If you have a high quality habitat where you might expect higher seed set you may try to increase seed production by taking cuttings from each plant as stated. Then if you have more room, take another set of cuttings from all plants, and repeat as long as you are not harming the plants and still have space in your planting site. This wil effectively capture the genetics of the original population, while increasing the population size and potential for seed production for storage or further conservation.
If this project is intended to stand to scientific rigor I would suggest labeling each mother plant, then label any cuttings from that plant with the same ID. This way people who might want to work on your project in the future will have what info they need, even if you don’t need it for your project. The small round aluminum tags stuck in the ground with a nail at each plant seems to be a good way to label them without seeing the tags from a distance.