If, after you've hired the number of Sidekicks you requested in your original post, you decide you want to hire more Sidekicks for the shift, you can do so by simply re-opening the request.
First, navigate to your live requests page and click into the job for which you want to request more Sidekicks.
You then just need to click the 'hire more Sidekicks' button (top right hand side under 'need more Sidekicks'), enter the number of additional Sidekicks you need, and select who you want to notify about your request (e.g. All Sidekicks or one of your personal Talent Pools).
Then simply hit confirm, and your request will be re-sent and available for all relevant Sidekicks to apply (note: Sidekicks who have previously applied for the job will be re-notified, however those that you have already hired or previously removed or declined will not).
You will then receive applications and hire new applicants as normal.
You can request more Sidekicks at any point prior to a shift commencing, however remember that the more time you allow, the more likely you are to receive your required number of Sidekicks!