As a starter, I would suggest you build 6 GPUs mining rig which less expensive than building 7 or more GPUs. The good thing here is that you can sell your rig once you want to upgrade to 7 GPUs and above. The following are hardware you need to build your rig and you're going to mine Ethereum for example...

Graphics Cards:
EVGA 1050 Ti FTW - Average Hash Speed: 14 MH/s
MSI GTX 1060 ARMOR OCV1 6GB - Average Hash Speed: 24.75 MH/s
Gigabyte GeForce GTX 1070 Ti - Average Hash Speed: 31.7 MH/s
Nvidia GeForce GTX1080Ti - Average Hash Speed: 36 MH/s

Motherboard
ASRock H81 Pro BTC R2.0
GIGABYTE GA-H110-D3A

CPU
Intel BX80662G4400 Pentium Processor
Intel CPU BX80662G3900 Celeron G3900
Intel 3.70 GHz Core i3-6100 3M Cache

Powersupply
EVGA SuperNOVA 850 G3, 80 Plus Gold 850W
Corsair CP-9020139-NA HX1000 1000W 80 Plus Platinum
Seasonic X-850(SS-850KM3 Active PFC F3) 850W 80 Plus Gold ATX12V/EPS12V

Storage/Hard Drive
SanDisk SSD PLUS 120GB Solid State Drive (SDSSDA-120G-G26)
Kingston A400 120GB
Silicon Power 60GB SSD S60 MLC High Endurance

RAM
Kingston HyperX FURY Black 4GB 2133MHz DDR4 Non-ECC CL14 DIMM Desktop Memory (HX421C14FB/4)
Corsair Vengeance 8GB (1x8GB) DDR3 1600 MHz (PC3 12800) Desktop Memory 1.5V
Ballistix Sport LT 4GB Single DDR4 2400 MT/s (PC4-19200) DIMM 288-Pin

With the above 6GPU setup, you will need also 6 PCI-E risers and rig case which you can buy online via Amazon or eBay. Or, build your own rig case, it's so easy simply go to youtube and search for 'mining rig case tutorials' and you'll find a lot.