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If two order automorphisms agree on $\mathbb{Q}$, their values at any real $x$ are the same supremum of the common values at rational points below $x$. This is the uniqueness argument for extension of an order automorphism from a dense subset; it does not require either automorphism to carry $\mathbb{Q}$ onto itself.

Their restrictions belong to ${\mathbb{R}}^{\mathbb{Q}}$, whose cardinality isThis gives the upper bound. The translations $x\mapsto x+r$, one for each real $r$, are distinct order automorphisms, giving the lower bound. Consequently