What does this sentence mean?
It just means ’eleven to nineteen’, but Swedes sometimes tell the time in 24h, so I’d guess it means ’11 AM to 7 PM’.
Can this mean that the time is 18:49?
No, that could possibly be elva i nitton, not that it's likely anyone would say that since we don't really use the numbers over 12 with i – I'd say elva i sju instead.
can it mean the score of some sort of sport?
No, for the score 11 - 19, we normally just say "elva nitton".
Could it be used in this context? "How many guests are you expecting? I don't know...maybe eleven to nineteen?"
I think you may be right. I was thinking about time (and then betting and scores, influenced by previous comments) and didn't consider range at all, which is I think the most straightforward of all contexts.