How many wives will a believer have in Paradise? Will he be able to do justice between them?

All perfect praise be to Allaah, The Lord of the Worlds.  I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

With regard to the number of a believer's wives in Paradise, Imaam Al-Iraaqi mentioned that the believer will have at least two wives from among women of this present life and 70 from the Houris, with no limit to the maximum number. 

It was narrated on the authority of Abu Moosa Al-’Ash‘ari that the Prophet, sallallaahu ‘alayhi wa sallam, said that there is a tent in Paradise made of pearl which is 60 miles in length. In it, the believer will have so many wives that they cannot see each other, given the big size of the tent. [Al-Bukhari]

It was also narrated on the authority of Ibn ‘Umar both, that the Prophet, sallallaahu ‘alayhi wa sallam, mentioned that if the lowest in station  among the inhabitants of Paradise wants to looks at his gardens, his wives and his bliss, it will take him a thousand years' journey in terms of this world. Moreover, the one who will be most honored by Allaah The Almighty will be the person who looks at His Noble Countenance in the mornings and evenings.  [At-Tirmithi (Ibn Hajar and Al-Albaani: Dha‘eef)]

As for your question, "Will he be able to do justice between them?," it is a meaningless and irrelevant question because the home of the Hereafter will be a home of justice and truth.  None of the inhabitants of Paradise will be worried about earning a living or otherwise.  Rather, all of them will live in happiness and bliss without any feelings of hatred, sorrow or grudges. 

Allaah Knows best.