Usually it's just the one employer involved. There isn't a limit on the number of respondents who would be, generally speaking, key employees involved in the case. Usually, key players along with the employer might be added, such as Upton. and as we see here, potentially, KS.
Once the ET is underway, however, an application to add another respondent would have to be made under the ET rules of procedure. In short, it's not a done deal that KS will be automatically be added. The applicant's (SP) lawyers would have to set out why they want to add her (KS), or someone else, at this time, and seek approval.