If a man has the ability to take one more then one then it is his right, but if he cannot then he should marry only one:
فَإِنۡ خِفۡتُمۡ أَلَّا تَعۡدِلُواْ فَوَٲحِدَةً
but if you fear that you will not be able to deal with them justly then marry only one
Furthermore a man does not have to seek permission from the current wife to take another. For part of the wisdom of taking another wife is to help the society in whole, for it is natural that women (in most societies) are more then the men, hence taking more then one (wife), can ensure that the society stays balanced and that women can find husbands, and Allah knows the full wisdom. Furthermore it would be natural for a women to be jealous and not want to share their husband, but this jealousy should not prevent the man from what is ok/halal/permissible for him to do. So basically to get back to the point, it is not a prerequisite nor an obligation to seek a wife's permission to marry another.
Main Sources: Islamic online university