How many PhD students are too many?
This is more of a qualifying remark: What your eventual target might be should probably be different from your current target. Advising students is a skill you have to learn and learning this skill will take time. So I'd recommend starting small until significant parts become routine. Also, more senior students might be able to show new students the ropes, so staggering might be useful too. 

There are of course a lot of other constraints, so don't take this as your only criterium.