Use count() in relational algebra
A syntax of aggregate operation in relational-algebra (according to [1]) is as follows :


  $G_1,G_2,...,G_n \hspace{2 mm}\textbf{g}\hspace{2 mm}
  F_1(A_1),F_2(A_2),...,F_m(A_m)(E)$
  
  where $E$ is any relational-algebra expression; $G_1,G_2,...,G_n$
  constitute a list of attributes on which to group; each $F_i$ is an
  aggregate function; and each $A_i$ is an attribute name.


Thus your query would look like :
$\pi_{name}(\sigma_{phone&gt;1}(name\hspace{2 mm}\textbf{g}\hspace{2 mm} count (phone)( Person)\hspace{2 mm}))$

[1] A. Silberschatz, H. Korth, S. Sudarshan, Database System Concepts, 5th Edition