How can I set to more than 100 sessions?

There is not currently. I would accept a patch to change it to 500 or better yet a text field combined with a form validation hook that does an is_int() check.

To get more than 100 concurrent logins, it appears that you could change line 372.

For example, to allow for 500 concurrent logins, change line 372 from:
$options = drupal_map_assoc(range(0, 100));
to
$options = drupal_map_assoc(range(0, 500));

Correct me if I'm wrong, but a quick run through the code, and quick test allows me to use 500. However, i haven't actually logged into 500 simultaneous sessions so I can't completely confirm this is the fix. However, I logged in with two concurrent sessions. I can verify that 500 shows up in the UI after saving the settings, and going back into to double-check.

That would do it, but 156 simultaneous logins? I'd rather have it be a validated text field instead of a dropdown if there's really a need for more than 100.

I wanted this issue to go away, so I increased the limit to 250.