There is no universal foam-board thickness that guarantees a model base will stay flat. Core material, facing, unsupported span, glue and the weight of scenery all affect the result.
Make a test base
Cut a sample with the intended unsupported span. Apply the same adhesive, paint and scenery layers as the finished piece, then let them dry fully. Check bending, curling and edge damage before cutting the full batch.
For a large or heavy terrain piece, consider a rigid backing compatible with the materials instead of stacking soft board and assuming more thickness solves the problem. Seal or finish edges with a compatible product; solvent-containing products can damage some foams.
Measure how much base height will be visible and whether that affects storage or tabletop play. Foam board versus cardstock explains the material choice. A successful light sample does not certify the base for a heavier display.