[BUG] Batch mode creates batches in a weird way

[Oloremo]

Description

Salt has the ability to run something in batches. I noticed that:

1. Minions split per batch is very weird
2. using batch sometimes leads to an exception

Setup
I have a cluster with 33 minions - an odd number of minions could be important here.

Seems like it's only reproducible while doing state.apply, running something like test.ping will work just fine.

The exception is the same as I reported here: #56273

Steps to Reproduce the behavior
I run something like:

 salt -b 10 \* state.apply formula.java test=True

And get this: https://gist.github.com/Oloremo/d1af8fef0410b2979584afa427fca555
Note that I removed all state output since it's irrelevant here.

Expected behavior
With 33 minions and batch 10 I'd expect to have 4 batches: 10, 10, 10, 3

Versions Report

salt --versions-report

(Provided by running salt --versions-report. Please also mention any differences in master/minion versions.)

Salt Version:
           Salt: 3000.3

Dependency Versions:
           cffi: Not Installed
       cherrypy: Not Installed
       dateutil: Not Installed
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
         Jinja2: 2.11.1
        libgit2: Not Installed
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.6.2
   mysql-python: Not Installed
      pycparser: Not Installed
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 3.6.8 (default, Aug  7 2019, 17:28:10)
   python-gnupg: 0.4.5
         PyYAML: 3.11
          PyZMQ: 18.0.2
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.5.3
            ZMQ: 4.3.1

System Versions:
           dist: centos 7.6.1810 Core
         locale: UTF-8
        machine: x86_64
        release: 3.10.0-957.1.3.el7.x86_64
         system: Linux
        version: CentOS Linux 7.6.1810 Core```
</details>

**Additional context**
Add any other context about the problem here.

[Oloremo]

Hm, it maybe works correctly just badly named. Should be something "max-concurrent"

I kinda guess it could work as "no more than N at once", so with batch 10 if 1 finishes and 9 are still running - salt goes ahead and launch one more minion since 9 < 10.

[dwoz]

@Oloremo I agree the name is not great but I doubt it could reasonably be changed now because of how long the feature has been around.

We should at least update some documentation to make sure this is more clear.