There are numerous standards for welding, there is not "a standard" to weld all materials in all thicknesses by all methods using all equipment.
First step, confirm if there is any quality control program or process at the facility. If so, review it.
Secondly, confirm what codes or standards are applicable to the finished product. Review those standards for any limitations on welding procedures, etc. The standards may limit the number of pieces that can be used.
Thirdly, confirm if your company has ever qualified weld procedures or welders per the standard in question. If so, your best options are to use procedures already qualified and welders already qualified.
If necessary, you can check with your welders and see what methods, thicknesses, materials, etc., they are comfortable using, set up welding tests to qualify the corresponding procedures and take it from there. The correct welding is that which can be done quickest, most economically, and fulfill all quality requirements, and may vary depending on the personnel and equipment available, as well as on the material and codes being used.
You also want to looking into weld examination (radiograph), and any forming issues (annealing or stress relief, etc.).
I think a lot of heads in that size range are seamless; I was thinking most non-seamless heads were made in just two pieces, but I'm not sure what the options are.
There are certain pre-qualified procedures available in some cases, also.
The question is very very vague, and more specific information may get you more useful answers.
