@Sandeep_Patel Right now the RX5700 is the best GPU for mining Eth (which is the most profitable coin to mine) - @bick0012 will agree with this I’m sure
If you want to buy 12 of them and run a single rig, you will need to think carefully about the rest of your equipment, particularly which Motherboard you want to use and what power supplies (you will need more than one - probably 2 x 1200W PSU). If you decide on a ASUS B250 Mining Expert mobo you will be able to fit all 12 cards and your PSUs will be easily attached without any other cable/connectors needed. You could also look at an ASRock H110 BTC+ Pro which will also accommodate all 12 GPUs but you will need additional connectors to get 2 PSUs hooked up to it. Research that and decide which way you want to go because there are others besides the two I’ve mentioned.
Be careful to note the compatible CPU and RAM for whichever mobo you decide on - both boards I mentioned will not work with latest generation CPUs so you will be looking for older generation 6/7/8 I think which will be cheaper anyway. Just go for a Celeron or Pentium CPU and 4-8Gb of RAM as these are not overly taxed when GPU mining. Don’t get sidetracked into buying a powerful CPU to try to CPU mine - there is no meaningful profit in it and it just increases the risk that your GPU mining will have problems.
Other hardware you will need includes PCI-E Risers (some advice on those is available on this site), a small SSD (60-120Gb is plenty) and a frame to put it all in. You will of course need access to a keyboard/mouse and monitor so that you can set up all your software.
Think (and research) how you want to mine as there are lots of different options. Firstly you can run Windows or Linux. On each of those you can mine directly through a pool to your own online Wallet using Phoenix Miner or Claymores Dual Miner. For the 5700, most people find that Phoenix gives better hash rates.
You could also consider one of the all-in-one sites such as Nicehash where your rig’s hashing power mines Eth (or something else if it’s more profitable) and you get paid in BTC every 4 hours. I use Nicehash and based on my 6 x RX5700 rig’s earnings, I would expect that a 12 x RX5700 rig right now would be earning you about $30 per day (for a few days last week it was $40 per day). If that keeps up you would pay for your $6000 outlay in about 200 days (if your electricity is free of course ).
That’s a few things for you to consider and research for now - there is still a lot more to learn and lots of different options that I haven’t mentioned. I’m sure you will come back with questions as you read and gather other opinions. Searching a few of the threads on here where people have asked about setting up new rigs would be a good place to start.