I'd like to start managing tournaments again this year, if everyone is happy for me to do so.
How many challenger tournaments should I manage to take my first ATP event?
It's up to you. When you've done 2-3 challengers and feel confident you can do a good job at a bigger stage, then you can ask for an ATP event.
may i start the thread for gbr futures now?
sure, if you don't have that much time next week