You can use
expression-based queries, however what you are asking to do is more than that.
In your first example, you're asking questions about an Artist, but the MC expression language works by asking questions about File Properties. So you can ask
Does this file have Artist = X? You want to ask
Does this Artist have N files?, but the expression language doesn't work that way. To think in terms of MC's expression language, you have to pretend to be a file, and as a file you can ask yourself
Does my property X have value Y?In your second example, you are again asking a higher-level question:
After running the expression, are there X numbers of files produced? And that answer cannot be known by the expression language because on a file-by-file basis, each file can ask itself questions and provide answers, but once the run through each file is complete, the expression evaluation is complete. There's no more opportunity to now have an expression segment that counts the previous results.
There are tricks you can perform using MC's Set rules for file display and Global Variables to do things like this, but for most users they are very complicated and difficult to manage.