Where can I see the classification?
CWC Rules v2019

If you mean the results it would be updated on cutoff date. Still in the forums you can sse matchlinks to see the result.

I would like to know about the "multi-team players" issue in the preparation round, could you tell me what the conditions are for participating in this stage?

It is the same as in WCL/ECL. In one season player can play only for 1 team otherwise all his games will be forfeited. There is grace period of 1 match to cover cases, when he joins some match by mistake, but if he joins other team for the second time, than games are forfeited.
But as this competition cant be added to chessteams.org, checking is not automatic and multiplayer cases have to be reported by admins in the forum.
Seasons 2018 and older follow: https://www.chess.com/clubs/forum/view/guidelines-for-the-matches
General
Cities World Cup (CWC) is competitions between city related teams played in a knockout format. It is played on a yearly basis.
In all other rules than described below it follows official rules for WCL including flag rule, penalties for closed accounts etc. (https://www.chess.com/clubs/forum/view/2018-wcl-rules), CWC2020 (https://www.chess.com/clubs/forum/view/wcl-2020-rules)
Schedule and minimum players
PARTICIPATION ROUND
- min 5 players
- April-September of previous year
- round-robin groups, number of groups and number of promoted teams be set up based on the number of participants
- 10 teams get promoted into 1/16 round
- scoring and standings - as per the section 9 WCL rules
- seeding: top 2 teams based on the current team power
PRELIMINARY ROUND (1/16)
- min 10 players
- October-January
- 1/8 round participants from previous season are automatically promoted to this round
- 6 registered teams with the highest power as of registration end are automatically promoted to this round
- seeding: bucket 1 (1/4 round participants from the previous season), buckets 2-4 based on the current team power, teams from bucket 1 plays randomly bucket 4 and bucket 2 plays bucket 3
1/8 ROUND
- min 10 players
- February-June
- the bracket is created at this stage / 8 matches
- seeding: bucket 1 (4 buckets based on the current team power, teams from bucket 1 plays randomly bucket 4 and bucket 2 plays bucket 3
1/4 ROUND
- min 15 players
- June-October
- A - Winner 1 vs Winner 2, B - Winner 3 vs Winner 4, C - Winner 5 vs Winner 6, D - Winner 7 vs Winner 8
SEMIFINALS
- min 20 players
- October-February
- Winner A vs Winner B, Winner C vs Winner E
FINALS and RUNNERS-UP
- min 20 players
- February-June
Match template
Match name: CWC2019 R1: Team 1 vs Team 2
(Example: CWC2019 R1: New York vs Delhi)
Match settings
Starting Position: Standard
Days per move: 3
Rating range: Open
Players: x+ (based on round)
Concurrent games per opponent: 2
Auto Start: No
Rated: Yes
Round results are calculated on the cutoff date including result corrections for closed accounts. Further changes are not reflected in the tournament in any way.
In case of a draw during play-off stage a team wins when:
1) has more individual victories in first 12 tables (24 victories max.) or
2) has more individual victories with black pieces in first 12 table (12 max.) or
3) has more individual victories with black pieces
4) team with higher power
5) random