Hello Xedieho77
You cannot reserve bike spaces on IR trains.
A prediction is not possible, all depends on the weather. On days with good weather the spaces are indeed limited. In case there are too many bikes, you might face a denied boarding. You have to stay flexible.
When travelling with IR26, be there early to secure yourself a bike space. As it starts at Basel SBB the chances to have a bike space is greather than when you would board at an intermediate station.
Furthermore, a Monday might not be as crictical as a Friday or Saturday/Sunday.
Kind regards,
Roland