Baha'u'llah had a total of four wives

Baha'u'llah had a total of four wives. His fourth marriage was to a woman named Jamālīyyih.

This marriage took place late in his life. Jamālīyyih was related to one of Baha'u'llah's followers, as she was the niece of his loyal follower Khadimullah.

Persons familiar with Baha'u'llah's affairs confirm that prior to marrying Jamālīyyih, he had married a woman named Gawhar in 1867, who bore him a daughter named Furúghiyyih. His other wives included Navvab, the mother of 'Abbas Efendi, and his cousin Mahd-i 'Ulya, whose eldest son was Mīrzā Muḥammad 'Alī.

(Extracted from Der Bahaismus: Weltreligion der Zukunft? : Geschichte, Lehre und Organisation in kritischer Anfrage (Studienbücher im Gespräch mit der Zeit) - Francesco-ficicchia)

...in his old age he married Jamaliyya, the niece of his faithful follower, Khadim Allah. It is said that all his wives survived him.

(Earl Elder's Notes published in his translation of Kitab-i-Aqdas)

https://bahai-library.com/provisionals/aqdas/aqdas147.notes.html

Also here,

An Historical Analysis of Critical Transformations in the Evolution of the Baha’i World Faith by Vernon Elvin Johnson

https://bahai-library.com/pdf/j/johnson_transformations_evolution_bahai.pdf

The Religion Of The Bayan And The Claims Of The Baha'is by Jalal S. Azal

https://archive.org/details/TheReligionOfTheBayanAndTheClaimsOfTheBahais

Some discussion here,
https://www.reddit.com/r/exbahai/comments/1gvx6th/help_further_sources_on_the_topic_of_bahaullahs/

Sobhi and Avarih have also mentioned Jamaliyyih in their books.

Result of multiple marriages:

The sad story of the events that followed the death of Baha’u’llah are related thus by Mirza Jawad, who had come with Baha’u’llah from Edirne, and had remained a faithful follower all through the years at Akka: “Alas, alas for what we see today! All these spiritual virtues and humane practices have undergone a complete change. Concord has been replaced by dissension, constancy by cruelty, and affection by enmity. Dissent and mutual avoidance have appeared in this community.....antagonism and separation arose between father and son, brother and sister, husband and wife, and so forth; nay, God be our refuge! even envy and hatred.” The cause of this dissension was, according to Mirza Jawad, “the love of self and seeking after supremacy” of Abbas Efendi.

(The Baha'i Faith: Its History and Teachings by William McElwee Miller)

https://www.h-net.org/~bahai/diglib/books/K-O/M/miller/Bahaism_Miller_1931.pdf