Thank you for your questions. Depending on the size and location of cancer, we would need millions to more than billions of them to localize and generate enough therapeutic effect.
Yes, they reproduce and multiply, which can be undesired for biohybrid bacterial agents, because reproducing means dilution of the synthetic components. That means new bacteria without enough magnetic material on them, causing loss of steerability. However, there are genetic tools that stop microorganism growth, which could be ideal here. Reproducing should also be under control because your injected dose could be in the millions range, but you could easily reach billions within a matter of hours, since the duplication time of microorganisms can be just minutes (e.g., around 20 minutes for E. coli). Additionally, bacteria should be removed from the host body after the medical task is completed, therefore another concept, which is named termination switches, could be also added to bacteria to terminate them after they had carried out their task through laser-triggered hyperthermia, antibiotics, or bacterial lysis.

All the best,
/birgül