2 things: field and report

Yes we have, but I think osmin would like to have this number integrated into the calulation, isn't it osmin?

If these persons reduce the amount of places/seats a dedicated field is required.
If not one of the custom fields can be used.

It could be one of the custom fields. Too bad that it's name couldn't be changed (as far as i know). It would be perfect if this field could be shown/displayed on report or even better - exported to a *.csv file.

The name can be changed by adapting the language files or using language overrides.
This is because an edit field to configure the name would support one language only.

To show custom fields in the list views you would need to adapt the view's tmpl file a bit. [strike](e.g. /components/com_jem/views/eventslist/tmpl/default.php)[/strike] (see below)
To prevent overwrite on next JEM update you should use template overrides mechanism.

EDIT:
The list itself is handled by
/components/com_jem/common/views/tmpl/default_events_table.php
Sorry