#Pと見てもSharp Pと読めない

while the # of #P is pronounced sharp (some textbooks also list pound or number as acceptable, but I've never heard the latter two used in real life).

#PはNumber P, ♯PだったらSharp-Pと読むのが自然な気がする. ちなみにGarey and Johnsonの本の索引を見ると#Pは「ナンバーピー」のようである。ともある.

追記:Garey and Johnsonを図書室で見つけたので索引を確認してみた. Nの項にNumber等と並んで#Pがあった.

Computers and Intractability: A Guide to the Theory of Np-Completeness (Series of Books in the Mathematical Sciences)

Arora and Barakの今度出る本だと, This chapter studies #P, (pronounced sharp p), a complexity class that captures this notion.と書かれている.
一応Number Pと読みたくなる理由はちゃんとある. Arora and Barakは, However, in many contexts, one is interested not just in a single certificate, but actually counting the number of certificates. This chapter studies #P...と書いてるし.