How can I determine the entire number of available blockchain outputs for each denomination?
Is there a blockexplorer that tracks this data or a bitmonerod command I can use to find the answer to this?
There is a bitmonerod command,
output_histogram [[mincount] maxcount], which lists (amount, instances) pairs of outputs on the blockchain. The output is sorted by instances.
output_histogram takes an optional first parameter, which is an instances count minimum cutoff. That is, it will display all amounts with at least that many instances. The second parameter is an optional maximum cutoff. The default is to display all of them. For instance,
outout_histogram 5 9 will display a sorted set of amounts for which there are between 5 and 9 intances on the chain, inclusive.
Note that this is rather slow as it reads a lot from the database.
The wallet uses this to determine which outputs can be used for the requested mixin (in that case, assuming a trusted daemon, the wallet will supply the amounts to check, which is a lot faster).