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If most of your routing can be placed on top layer and have mostly solid ground plane sections on the bottom layer, 2 layer boards is most suitable for you which will not cost so much.
If your design is a semi-high speed design, then a four layer will be better. A lot of this comes down to how much resolution you need from those ADCs. If you truly need 16 bits, then I think a 5% performance increase from a four layer board is worth it.
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