The 20% number is part of the initial 3-year pilot program, not necessarily the ultimate goal. That's also the mandatory minimum, with agencies "strongly encouraged" to release as much as possible.

See the policy [0], linked from the launch announcement [1].

[0] https://www.whitehouse.gov/sites/default/files/omb/memoranda...

[1] https://www.whitehouse.gov/blog/2016/08/08/peoples-code