I'll often run 4 or 5 agents in parallel. I review all the code.

Some agents will be developing plans for the next feature, but there can sometimes be up to 4 coding.

These are typically a mix between trivial bug fixes and 2 larger but non-overlapping features. For very deep refactoring I'll only have a single agent run.

Code reviews are generally simple since nothing of any significance is done without a plan. First I run the new code to see if it works. Then I glance at diffs and can quickly ignore the trivial var/class renames, new class attributes, etc leaving me to focus on new significant code.

If I'm reviewing feature A I'll ignore feature B code at this point. Merge what I can of feature A then repeat for feature B, etc.

This is all backed by a test suite I spot check and linters for eg required security classes.

Periodically we'll review the codebase for vulnerabilities (eg incorrectly scoped db queries, etc), and redundant/cheating tests.

But the keys to multiple concurrent agents are plans where you're in control ("use the existing mixin", "nonsense, do it like this" etc) and non-overlapping tasks. This makes reviewing PRs feasible.