When TR let 2000 headcount go in the last redundancies in Nov 2016, they took a $212 million severance charge - this can be found in the 4th quarter results.
Therefore taking the $500 million one of charge being forecast over the 3 years to 2021, we can make a guesstimate that they intend to let about 4500 headcount go over the next 3 years. 1100 in this first tranche (as per the 5% number given by David Craig), and another 3400 approx over a time range until 2021.
A 20% chop overall. New headcount across the cheaper regions could mitigate that.