Assalamualaikum ustaz. How many of the companions that joined Rasullullah PBUH for hajj?
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
In the evidences the word jam’un ghaffir is used which means a very large number that even some companions from Yemen and other places also joined him.
Some say 100 000, but the famous opinion is 124 000. We include parts of a paragraph from the book al-Rahiq al-Makhtum:
On the eighth day of Zulhijjah which is on tarwiyah day, Rasullullah PBUH heads towards Mina. There, Rasullullah PBUH performed the Zohor, Asar, Maghrib and Isyak and Subuh, which is the five prayers, then the Prophet PBUH waited a bit until the sun rises. Then, Rasullullah PBUH heads to Arafah. As he arrives in Arafah, Rasullullah PBUH found a tent erected and prepared for the Prophet PBUH, and he stops over there. When the sun passes the zenith, Rasullullah PBUH called his camel, al-Qaswa and ride it to the middle of the valley (Uranah). There, there were 124 000 or 140 000 people gathered. Rasullullah PBUH gets up to deliver his sermon. 
Hence, those who state 124 000 also relate it with the number of prophets and messengers total of 124 000. Wallahu a’lam.
Let us all supplicate that may Allah grants us all accepted worships and a mabrur hajj and umrah.
 See al-Rahiq al-Makhtum, pg.801