In order to justify why the Prophet ﷺ would make a very bold declaration in front of tens of thousands, and perhaps even close to 100,000 [though this is fairly disputed], an attempt is made to claim that the discontent against Ali ibn Abi Talib عليهم السلام was by a large number of men who had begun a massive furore.
However, the fact remains that the battalion taken by Ali ibn Abi Talib عليهم السلام to Yemen contained around 300 men as per the majority of historical sources. Furthermore, it is illogical to claim all 300 showed discontent but rather, a percentage or fraction of the army did so.
In the following tradition from Saheeh-al-Bukhari  we can see that a few individuals at first complain against Ali ibn Abi Talib عليهم السلام and are instantly reprimanded. It is important here to note that the reply of the Prophet ﷺ to Buraida varies in Saheeh al- Bukhari, and two variant versions are given, and hence no-one has a right to claim any one tradition. It is likely the addition of ‘Whomsoevers Mawla i am, Ali is his Mawla’ was a confusion as this statement had been said at Ghadir given the various versions can not be reconciled.
The Prophet (ﷺ) sent `Ali to Khalid to bring the Khumus (of the booty) and I hated `Ali, and `Ali had taken a bath (after a sexual act with a slave-girl from the Khumus). I said to Khalid, “Don’t you see this (i.e. `Ali)?” When we reached the Prophet (ﷺ) I mentioned that to him. He said, “O Buraida! Do you hate `Ali?” I said, “Yes.” He said, “Do you hate him, for he deserves more than that from the Khumlus.”
We also find another report  demonstrating it was a few individuals at first who opted to complain:
“The Holy Prophet sent a group of army under the command of Ali ibn Abi Talib, after the war (Imam) Ali reserved one female servant for him. Among the companions four of them discussed negatively about Ali ibn Abi Talib and they decided that they will complain to the Holy Prophet,…When the above mentioned group of army return back they went to see the Prophet, one companion stood up among those four and he complained about Ali ibn Abi Talib. The Prophet turn his face on the other side then second companion stood up and also complained, then the third companion stood up and complained, the same thing to the Prophet then the forth one stood up and complain the same thing. The Prophet look towards then and we could see from the facial expression of the Holy Prophet that he was angry. The Prophet said “What made you to complain about Ali ibn Abi Talib, surely Ali is from me and I am from Ali and he is the Wali(Master) of all the believers after me.”
Although there is a debate about the authenticity of the above, it adds to historical evidence which suggests that it was not the entire army which showed resentment, but rather a percentage of the force of 300 who were not left to their thoughts but instantly reprimanded and corrected by the Prophet ﷺ.
Some have used the following report from the Seerah of Ibn Ishaq  to suggest it was the whole army or the vast majority who complained:
“The army showed resentment at their treatment…when the men complained of Ali,the Apostle arose to address them and he (the narrator) heard him (the Prophet) say: “Do not blame Ali, for he is too scrupulous in the things of Allah, or in the way of Allah, to be blamed…”…Then the apostle continued his pilgrimage, and showed the men the rites..”
Linguistically, one can use ‘the army’ to represent a percentage among the a small force of 300 [even if we assume there was another battalion of similar number], even fifty individuals complaining may linguistically be rendered in this way. It was far from a furore and a fitnah, but rather an issue held by a percentage from among a small battalion who were from Medina and knew well the status of Ali ibn Abi Talib عليهم السلام, and were corrected and reprimanded right away as to the correct action of Ali ibn Abi Talib عليهم السلام in how he handled the Khums, which would have thus closed the matter and made there absolutely no need for there to be any further declaration on this particular issue. Furthermore, even if a small number of individuals came to complain and if this number increased, it explains why the Prophet ﷺ after addressing individuals decided to ‘rise’ to address the people as a whole.
 Sahih-Al-Bukhari 4360, Book 64, Hadith 377. Vol. 5, Book 59, Hadith 637
 Khasa’is Ali- by An Nasa’i
 Ibn Ishaq, Seerah Rasool-Allah, p.650