The typical range is 10-20%, and largely depends on your hiring needs.
Best practice is to plan out your key hires and equity grants for the next 12-18 months and create a plan accordingly. If you are planning to hire senior executives, consider a larger pool. If your team is staying small, a small pool works well.
You can always increase the size of the plan later, so aim to make the plan just large enough. If you hit the cap before your next fundraising round, adding more space requires speaking with your lawyer to amend the plan and gathering board and stockholder approval.