But I think you can fill up the maximum ranch level every day. By depositing 20, you unlock the level that can hold 50; by depositing those 50, you unlock 100 and so on.
I don't see anything that says 20 per day, though.
This is how I think it works. The ranch works on a day cycle, so it grows once you hit certain levels on the following day. Assuming that is true, it looks like we'll be able to hit the maximum level of 1000 pokemon in 20 days and get that Mew. Assuming this is true, the first legit Ranch Mews should be obtainable on the 28th of this month.
EDIT: Actually, this is assuming the ranch can only go up one level per day. Assuming that the level can increase directly with the number of pokemon transferred over, the ranch could hit the maximum level much more quickly. If that is correct then we could see this progression:
Day 1: Add 20 pokemon
Day 2: Ranch goes up to Level 5
........ Add to 50 total pokemon
Day 3: Ranch goes up to Level 8
........ Add to 100 total pokemon
Day 4: Ranch goes up to Level 11
........ Add to 250 total pokemon
Day 5: Ranch goes up to Level 15
........ Add to 500 total pokemon
Day 6: Ranch goes up to Level 20
........ Add to 1000 total pokemon
Day 7: Ranch goes up to Level 25
........ Trade for Mew
This is, of course, assuming that you get to pick Mew from his trades. If that is true, then the first legit Mews should be popping up as early as the 15th or 16th. Wash, rinse and repeat and we could have a wide variety of Ranch Mews available for trade by the end of the month.