First time using Frontier, empty flight!
 

I am in the process of booking a flight for next month. I leave Milwaukee, with a connection in Denver. Then fly Denver to Billings. Both of these flights show exactly one seat taken! Now I am all for having more room on a flight, but I am more worried about the flights being cancelled.
What's up? Should I be worried?

You can't tell by just looking at a seat map. Remember, the lowest, "economy" fares don't allow advance seat selection until checking in within 24 hours of flight. It is highly unlikely that only one seat is sold on either of those flights, although at this time of year the flights probably won't be too full.

89% load factor
 

In addition to post abt seats filling up on classic fares 24 hrs prior, their load factor is an industry leader - nearly 90% last month. Lynx is notably lower, but bet that your Milwaukee flight is going to be full.

I thought the same thing when booking a flight from DEN to LAS and it made no sense to me at the time. The actual flight was completely full.

If you buy the cheapest ticket, "economy" you will only be able to select a seat at check-in. You can do it online within 24 hours. My recommendation is to check-in on line (choose the "view seat map" option) and select your seat even if you can't print a boarding pass. They can duplicate your boarding pass at the ticket counter but if you don't select a seat at all you might end up in a middle if you wait until you get to the airport. The other option is to buy a "classic" ticket which allows you to pre-reserve your seat at any time.

The way to find out how many seats are ACTUALLY reserved is when you select 'view seat map' make sure you change your selected fare to Classic Plus. That will show you the actual seat map as Classic Plus gives you access to the most seats to pre-reserve.