For extendability we save up space for upcoming attendants, so we take a fixed value of 400 participants and hope there will never be more Pokemon characters who would like to attend such an event

I came up with 200 because I thought of the following system: battling more than 10 rivals in this 18 days before the finals would be kinda exhausting, but with 16 groups (like in soccer world championship) we would have at most 160 participants. That seemed kinda small to me, but it would be a backup plan. My idea was we have 8 groups of 25 people, but with 5 people in a subgroup. All people in a subgroup battle each other and the one with most wins goes up to the "main" group, all others are out. Now the winners of the five subgroups battle against each other in the second phase and the two with most wins go into the finals. So we have 8 fights for each participant (who won the first phase) – which holds a nice similarity to the 8 gym leaders – and 60 fights per group, making 480 fights during the preliminaries and thus 26 to 27 fights per day. Judging fom the anime, a fight would hardly go 20 minutes in average (depending on the rules) and with two fights simultaneously and 45 minutes advertisement in general we would get 5.5 hours of entertainment each day. Sounds reasonable to me.
Now I post the groups as far as I had (randomly) planned them yet. A "!" before a person means he/she is the winner of the subgroup and a "(1.)" or "(2.)" means he/she is the first or second in the group respectively. And indeed there are "empty" cells ranging from 1Aa to 8Ee, where the number indicates the group, the capital letter the subgroup and the small letter the position within.
Group 1:
Group 2:
Group 3:
Group 4:
Group 5:
Group 6:
Group 7:
Group 8:
For better planning we may increase the daily fights, then we have 16 days of fights, maybe seventeen when we make a break between first and second phase
Timing of the fights (planned in that way no trainer has two fight two times a day during phase one):
1. day: 1Aa vs. 1Ab & 2Aa vs. 2Ab, 1Ba vs. 1Bb & 2Ba vs. 2Bb, etc., 1Ac vs. 1Ad & 2Ac vs. 2Ad, etc., 3Aa vs. 3Ab & 4Aa vs. 4Ab, etc. (30 fights)
2. day: 3Ac vs. 3Ad & 4Ac vs. 4Ad, etc., 3Aa vs. 3Ae & 4Aa vs. 4Ae, etc., 5Aa vs. 5Ab & 6Aa vs. 6Ab, etc.
3. day: 5Ac vs. 5Ad & 6Ac vs. 6Ad, etc., 5Aa vs. 5Ae & 6Aa vs. 6Ae, etc., 7Aa vs. 7Ab & 8Aa vs. 8Ab, etc.
4. day: 7Ac vs. 7Ad & 8Ac vs. 8Ad, etc., 7Aa vs. 7Ae & 8Aa vs. 8Ae, etc., 1Aa vs. 1Ae & 2Aa vs. 2Ae, etc.,
combinations ab, cd, ae in every subgroup done
5. - 8. day: same scheme with bd, ce, ad
9. - 12. day: same scheme with be, ac, de
13. day: 1Ab vs. 1Ac & 2Ab vs. 2Ac, etc., 3Ab vs. 3Ac & 4Ab vs. 4Ac, etc., 5Ab vs. 5Ac & 6Ab vs. 6Ac, etc., 7Ab vs. 7Ac & 8Ab vs. 8Ac, etc. (40 fights) + end of first round
Winner of subgroubs will be denote by simply leaving the small letter
14. day: 1A vs. 1B & 2A vs. 2B, 3A vs. 3B & 4A vs. 4B, etc. until & 8A vs. 8B, the same again with C vs. D, then A vs. E (32 fights)
15. day: B vs. D, C vs. E, A vs. D
16. day: B vs. E, A vs. C, D vs. E
17. day: B vs. C (8 fights) + end of second phase and preliminaries
18. day: calm day and reeeeeelaaaaaaax
The finals are hold as following (where N.1 and N.2 stand for first or second winner of group N respectively) (read this columnwise too)
The winners of the fights standing directly under each other will fight in the quarter finals, then the two winners of a column in the semi finale and the two winners of this in the finale after third place was found out.
With this, the finals are as "predicted" in the first post