The number of miners doesn't really impact your returns, it's more your power, miner maintenance, space costs of the miners.
On average, the two best in class miners as of this post are the Canaan Avalon 741 and the Bitmain S9.
The 741 costs $800(US) and produces 7 TH/s at 1200 watts (~29 kWh of power.)
The S9 costs $1500(US) and produces 14 th/s at 1500 watts (~36 kWh of power.)
Both will require PCIe cables and power supplies, which are an addition $100-$150 or so.
Neither will be in stock for the rest of the year, so if you want them NOW, you'll have to pay a larger mark up on eBay or to a forum member.
Those watts translate almost 100% into heat, so the more miners you have, the more heat you'll have to deal with (either exhausting/venting it, or using active cooling like AC)
Each miner is a little bit bigger than a shoe box, so you'll need racks to store them - and they're pretty loud, putting out about the sound of a vacuum cleaner.
Many people use a separate space to store them - whether its their garage, a shed they've built/modified, or a warehouse.
The S9 can be unreliable, and while under warranty, your only option is to ship it back to China for them to repair it, which takes weeks at least, months at most.
The Avalon is a little more reliable, but less overall hash rate.
Finally, the difficulty increases every 2016 blocks, or roughly two weeks. This means your miners get less effective all the time. Eventually, new miners come out to replace them, so you're hoping you've made enough coin to pay for those so you can keep up.
Since you don't say what you consider a "considerable" return, I'll advise you buy 1000 S9s and 2000 A471s.
Thanks's a lot for the realistic insights.
Wow! Mining is a very expensive investment. 1000 S9s and 2000 A471s is way beyond my projection and capacity.
Add to it the energy cost where the country I live has one of the highest kWhr rate.
I really need to rethink my decision on mining.
Thanks once again for the lengthy but very insightful response!!!
