How can I specify quantity of tickets on event participant import?
There isn't a way to record multiple tickets directly - CiviCRM expects every event participant to have a contact record.

The simplest solution is to create a custom field associated with Event participants (or even with the participant role) that indicates the number of tickets booked. The only issue there is whether you can import the custom field - it should be possible, but I admit that I haven't tried this.

One way to get round the issue would be to import a duplicate event participant for the person who booked - but that could be confusing, as there's no way to tell when a duplicate is intentional. (The import process should allow you to skip duplicate checking).

A more complicated solution would be to create a new contact record for the guest (and create a relationship between that record and the person who booked), then import the event participant for that guest record. You can then go back in after the event and add in their real details (assuming you find out who the guest is).

This is assuming you're not also importing event fee information - it's not something we currently track, so I can't really help there - it's possible there are solutions available involving event fees/contributions.