Is there a tool that I can use to search the blockchain for transactions sorted by number of mixins?
For example if I wanted to find all transactions with exactly 10 mixins how would I do it?
You'll have to write some code for this.
There is a BlockchainDB::for_all_transactions function which can run a callback through all the transactions in the db. See the blockchain_dump.cpp file for an example. The callback can simply output the hash for all the transactions with a mixin of 10 (mixin being one less than the number of inputs in each vin array element). Note that those elements don't necessarily have to have the same size (ie, inputs can have different mixins).