How many PhD students are too many? I'm a new assistant professor in the UK in Electrical engineering. I'm in the process of setting up my lab and I work in a "hot" area. Thus, I have the chance of getting good students funded by industry or funding bodies.

I know of professors with 10+ PhDs and others with 2-3. The group I did my PhD in was in the second category while my postdoc in the first.

However, I am wondering, what should be my target? Any studies have been made? Any personal experience?

I currently have three PhD students.
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.