We try to be around 1m potential reach per AdSet. So for mobile apps the Saved Audience or Lookalike has to be at least 2,5m - more for many countries where iOS has a smaller market share.
Our reason is less the minimum events requirement Facebook has for its learning phase - 50 purchase events can be accomplished with much smaller audience in decently monetized mobile games. We rather want to make sure an AdSet can run with significant spending after the learning phase is completed to benefit from the completed learning and potentially Creative iteration.
A related observation: even with large audience sizes Facebook prefers to impress the users it considers most likely to convert according to your optimization goal multiple times rather than quicker utilize a larger share of the audience. So you'll see the frequency go up to around 2 pretty quickly even if you've only reached a fraction of the audience size. Most of our AdSets reach a cumulated frequency of ~2-2.5 within 7-10 days, and then the share of newly impressed users vs. users who are impressed again per day stays relatively flat at 25-40%. For that reason we stopped using much larger audiences than the approximately 1m.