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Robin Chapman
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Uncountably infinite as long as $d\ge2$.

We can solve the boundary value problem for countinuous functions on the unit sphere $S$. So we get a linear injection from $C(S)$ to $H(B)$. Now the dimension of $C(S)$ is uncountable. The unit interval $[0,1]$ embeds in $S$ and by Urysohn's lemma we get a linear surjection from $C(S)$ to $C([0,1])$. To see that $C[0,1]$ has uncountable dimension, note that the functions $f_t:x\mapsto e^{tx}$ are linearly independent for $t\in\mathbb{R}$.

Robin Chapman
  • 20.8k
  • 2
  • 66
  • 81