How nickel plated copper strip can be welded? What is the optimum thickness of nickel layer?
It is difficult to give you exactly the answer you are looking for perhaps because we don't know what welder you have, but I can give you some insight from my own 18650 nickle plate welding experience, and perhaps direct you to a place where you can find more answers. 

First things first, it is very difficult to find a hobbyist welder that is worth a damn. You don't mention what welder you are using, nor do you mention if you are using wire-handheld-leads, but I was using a sunkko 709a. Generally users are always frustrated with this welder,and most of the others in the same $400 price range. It doesn't consistently weld the nickle to the 18650 well. The reasons for this are that they are using too think of nickle strip for the welder to weld. The welder can only push so much current through the system, so if your nickle is very thick, not only does it require more heat but that heat requires much more current to generate as the resistance of the nickle is less. 

One way to combat this is to use a 220v welder. I live in Canada and we have 120V commonly available, as well as 240v for dryers etc. The 220 Sunkko supplied with 240v would weld up to even 0.25mm nickle, even with the hand held leads, but the welder needs cool down periods or it will over heat. In general I would not recommend the welder. 

Another solution is to use thinner nickle and put multiple layers on. I used 0.1mm nickle and had much more success, and less troubles with overheating. If you want to use copper you will have to go much thinner still as the resistivity of copper is less. 

One other trick is to have a slit in the nickle/copper right in between the electrodes of the welder. ".|."  The slit forces the current through the 18650 material instead of straight through the copper/nickle. It does help for sure. 

A more powerful welder can also do thicker materials, but it is not really a great idea to do that to an 18650 as some of them are very thin cans. The Sanyo ncr18650ga cells, for example, are super thin and will quickly have a hole burned through their can if you weld them at higher power. 

A more powerful or better welder is useful in that you can set it to lower setting and get consistency. There are also some that monitor the amount of current and number of pulses in each weld and will alert you if you have a bad one. Though I haven't bought one yet, the next welder I buy will be this one. 
https://www.youtube.com/watch?v=rQnODV4VQjU
I have only heard good things about it. 

Good luck.