I don't know. But just guessing I would say as many as you like as long as the heat is sufficiently being evacuated from the desired area and that the heat isn't so great that it causes premature heat failure to the extraction fan. I assume you will be using an inline fan. Of course you know these fans work better when pulling air, so it may be subject to excess heat. Something to consider would be figuring out how much volume there is in your desired enclosed reflector, and how many you want to use. You will also have to take into consideration what size pipe you will use.
volume in cubic inches= lxwxh
then divide this total x 5 to = cfm
(cubic feet per minute)
to figure out the cfm rating of your required fan. I would use the next strongest fan to be safe.
I am a licensed Journeyman Plumber (unlike "Joe the Plumber" what a dipshit) and I have to figure out shit like this for my job all the time. Good Luck.