Does a transaction with 50 inputs have 50 different hashofthingtosign, one for each input, or does it just have one that is used for all the input scripts?
It is different for each input. Each input's hash preimage contains that input's previous output's scriptPubKey in that input's scriptSig. All other inputs have empty scriptSigs. This means that the preimage for each input is unique, and thus has a unique hash.