how can we change or remove the topic above the cb lists (X has Y members)?

It's a language string so you can use language overrides shown below to change it to say whatever you like or translate it to empty string to remove it.

www.joomlapolis.com/blog/kyle/18701-language-overrides-have-arrived

The language strings specifically for it are as follows.

Key: SITENAME_HAS_TOTAL_REGISTERED_MEMBERS
String: [SITENAME] has %%TOTAL%% registered member|[SITENAME] has %%TOTAL%% registered members

Key: USERS_COUNT_MEMBERS
String: %%USERS_COUNT%% member|%%USERS_COUNT%% members

And another question: Could it be possible that every cb list has another topic?

Yes, the above language string has a language key that's userlist specific. Example as follows.

From: SITENAME_HAS_TOTAL_REGISTERED_MEMBERS
To: USERLIST_{LIST-ID-HERE}_TOTAL_REGISTERED_MEMBERS
Example: USERLIST_1_TOTAL_REGISTERED_MEMBERS

From: USERS_COUNT_MEMBERS
To: USERLIST_{LIST-ID-HERE}_COUNT_MEMBERS
Example: USERLIST_1_COUNT_MEMBERS

With the above Example that string would only change for the userlist with an id of 1.