Should a Muslim perform a single Istikhara prayer for several university majors, or a separate prayer for each major?

The meaning of Istikhara is seeking the better of two matters. It is originally for one matter, so it is advised to consult with experts to determine a single matter for which to seek guidance from Allah. If Allah does not ordain this option for you, then move to the next option and perform Istikhara after consideration and consultation. It is permissible to repeat Istikhara for each available option.