Praise be to Allah.
Shaykh Ibn Baaz (may Allaah have mercy on him) said:
With regard to what some people do, doing a lot of ‘Umrahs after Hajj from al-Tan’eem of from al-Ji’raanah etc., we have stated above that there is no evidence that it is prescribed to do ‘Umrah after Hajj, rather the evidence indicates that it is better not to do that, because the Prophet (peace and blessings of Allaah be upon him) and his companions did not do ‘Umrah after they had finished Hajj. Rather the ‘Umrah that ‘Aa’ishah did from al-Tan’eem was because she had not done ‘Umrah with the people when they entered Makkah because she was menstruating. So she asked the Prophet (peace and blessings of Allaah be upon him) to let her do ‘Umrah to replace the ‘Umrah for which she had entered ihraam from the meeqaat, and the Prophet (peace and blessings of Allaah be upon him) let her do that, so she did two ‘Umrahs, the ‘Umrah that went along with Hajj, and this ‘Umrah done on its own. Whoever is in a similar situation to ‘Aa’ishah’s, there is nothing wrong with him or her doing ‘Umrah after completing Hajj, following all the evidence and to make things easier for the Muslims.
Undoubtedly if all the pilgrims were to do another ‘Umrah after completing Hajj, apart from the ‘Umrah they did when they entered Makkah, this would be too difficult for them all, because there would be too much overcrowding and too many accidents, as well as the fact that it goes against the teachings and Sunnah of the Prophet (peace and blessings of Allaah be upon him).