It depends on the area you setted, normally it can expend to 10 dogs with extra collar, but too many collar will affect the signal, we recommend three collars at the same timw.
Yes, it is safe and harmless. All our products are comply of ECMA standard. As long as you use our products according to the manual,it is totall safe.
We recommend charging once per week.
No. The collar may break or the Contact Points will pull too tightly against your pefs neck.