24Bay R740xd use SAS expander for backplane. As it use SAS expander RAID controller can support more than 8 drives. 
Hello! We are choosing a server to buy and the authorized seller offers us a R740XD with perc h750 and 24 2,5 hdd.  There are some documentation on these controllers here which says that maximum amount of  disks per h750 is 8 (docs). But the seller insists H750 can work with 16 or 24 disks. Please, help us find the truth please.  
Perc H750 Physical disks amount per controller