There are more than hundreds of missions in GTA San Andreas definitive edition, well only 86 are the story missions and 14 missions are optional, and the rest all are side missions and other missions that are not related to story missions. Means 86 missions are mandatory missions if you want to complete all story missions and 14 missions are optional missions, which means it is totally up to you whether you want to complete or not. optional missions contain story characters, such as Wu Zi Mu and the Triads. Rest all missions are sided missions or other missions.
GTA San Andreas has more missions than GTA 5, which is one of the smallest in the series of GTA games.
After a completing certain number of missions, about everything gets unlocked in the world of GTA San Andreas, otherwise, if you go direct to someplace, you will gain a wanted level.
Here is the GTA San Andreas Definitive Edition Missions Guide video playlist link – Click here