without sounding too rude, but we know you are planing to charge for your services, and you asking a question like this?
I really hope your clients don't surf ClubSNAP.
please don't ask for quick fix.
it could be you shooting with flat batteries,
or you shoot on ISO 100
or you shoot flash with ceiling bounce in a big ball room with 5m ceiling or low ceiling painted in black
or you shoot everything in f11
or you shoot your subjects at 10m away.
how to answer you when we even don't know what is going on?
anyway, what you should do now is to brush up your basic photography.
1) Define what you meant by 'normal' professional photographer (is there a 'special' professional photographer?)
2) Define what you meant by your flash only had one shoot, you mean, you shoot 1 time and the bulb blow and you need to change the bulb or the entire flash exploded and you need to change the entire flash?
As for how many flash a photographer has depend on how many he or she needed for the job to be done.
As usual... no head no tail question... how you aspect anyone to answer?
It's quite difficult to guess what you really need. Let me give a try on the first part of your post.
I'm not a Pro, but love to watch them work.
Normally I see them only use one, but with reflectors held by their assistant.
Having saying this, I strongly believe they have at least another one backup Flash unit in their bag. It would be crazy for a pro to go on assignment without backup.
On rare occasion, I saw some are using 2 flashes. One mounted on camera while other mounted on flash stand with diffuser and he did have reflectors like previous one too. According to him, diffuser is better than umbrella, since umbrella can topple easily under strong wind. And the person I asked does have the 3rd Flash in his bag as backup.
I hope this help, if you are not referring to repeating flash.