Question on querying agents

Now that y’all have seen my query, as well as a description of how I picked literary agents to query, here’s a question to authors who have been through this already:

Should I query all 12 agents at once? Or would it be smarter to query the first half and wait for feedback?

Here’s the advantage of going the latter route: If I don’t get any bites, I can change the query and proposal according to their feedback and then approach the second half of the group.

Thoughts?