Enabled/Disabled state should be maintained on import of a new user. Are you saying that disabled users are becoming enabled?
When i Upload the csv file with all the user using the web interface, there were problems with some of te users I have to upload the files several times. In the files there are less that 400 users but it seems to large for the system.
How many you can import at once depends on your servers hardware. I recommend importing in batches of 200. In CB 3.0 we'll be improving this with a processing queue so that we can automatically break up the import for you, but for now you have to do so manually as it was never intended be used as a means of migrating an entire install when originally developed.
A email is sent to all the users that I upload to new environment. I don´t want to send an email. I see an option that says: ignore emails for new entries. How can i do that by default?
See the parameters under the drag and drop/click file field. There you can completely suppress emails.