Change number of shifts wording to be more certain

Luke Duncalferequested to merge
patch-19463 into master

This change comes out of my own uncertainty about how many shifts I was expected to choose.

I clarified with @cdu1 in Slack that we currently have around 100 backend engineers.

This obviously means either:

  • the numbers in this doc will need to be updated every so often, or
  • we should link to somewhere that grabs this data from our team.yml page in future

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