There are 187 objectives in total in GTA San Andreas which are REQUIRED for 100% completion.
There are various side missions and objectives which are not required, and there are various objectives included in some of the objectives listed below which are not required separately, but are required altogether.
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For example Chiliad Challenge 1 gives you no extra percent, neither does the second or third on their own, however after winning all three races you'll be awarded with the percentage. You get no percent for each single car, but you do get percent for the fully completed list.