Q. Will there be always some souls for in this material world or will there be a moment in eternity when there won't be any material world. For I heard that for the ompleteness of Lord Krishna there is need for material world.
The Lord is in fact full, and one part of that fullness is the existence of His external energy. However, Krishna can withdraw that external energy and keep it in a dormant condition and still maintain His fullness. There is also a clear statement found in Srila Prabhupada's teaching, that there will be souls eternally existing in this material world if they so choose to remain here. There is no necessity that they must return to the spiritual world. Were that to be the case, then free will would not be a reality, and this would be a fundamental contradiction in Krishna conscious understanding.
Q. We as a soul are said to have minute amounts of all the qualities of Lord Krishna. Then why is that some qualities dominate. Is it to do with the modes of material nature?
Yes, this has to with both guna and karma.