as salam alaikum
the Qur'an says: "Surely Allah does not forgive that a partner be associated with Him but He forgives everything else to whomsoever He wills" (4:48). On the other hand, He says: "Do not despair of the mercy of Allah. Indeed He forgives all sins: (39:53).
To understand this, we need to understand that Allah forgives anyone who believes in Him: "Surely I am Forgiving to him who turns in repentance to Me and believes and do righteous deeds and sticks to the guidance" ( (20:82).
Shirk is not something to be forgiven by Allah in the sense that it is the mushrik to put himself out of Allah's mercy. A wrongdoing committed against yourselves can be forgiven through Allah's mercy, a wrongdoing committed against another person also can be forgiven in a similar way, as long as we are pure enough to turn to Him and be receivers of His mercy. Shirk however cannot be forgiven in such way because by limiting Allah's presence in your heart, you are limiting Allah's mercy on you as well. It is reported in a hadith qudsi that Allah said:
أنا عند ظن عبدي بي
"I am as My servant thinks I am"
It is only through faith and by fully relying on Him, that we can uproot shirk and hope in His mercy and Forgiveness.
In conclusion we can say that there is no limit to Allah's mercy as long as the servant does not limit himself and turns away from it.
With prayers for your success.