In general, there is a concurrency limit of 300 per migration endpoint. So, assuming you have only one migration endpoint, yes. You can have migration batches running (i.e. running, queued, paused), each utilizing different migration endpoints, increasing throughput.
* If we're running staged migrations is there a hard limit of how many can sit paused at 95% in my migration console? No, it can be more.
* I can see there is a max 300 concurrent batch limit but do paused/staged migrations count towards that 300? No - it's a concurrency limit for the mailboxes concurrently being migrated or periodically synced, through an endpoint.
* Also as I read it there is no limitation to the number of mailboxes inside a batch? So best practice certainly wouldnt be to create a 1:1 user - batch process? The CSV is limited to 2000 entries. Batches are formed to group users you want to migrate together. In practice, groups are usually formed by dependencies (boss/secretaries), business units, geographical location, etc. It's a lot of overhead to create a batch per user, but if that works for your organization.
Best Response confirmed by
Mike Rowland (Occasional Contributor)