It depends on the resort and booking category you booked. If you book a 1BR that has a second bathroom (Kidani and BLT at WDW, not sure which, if any, non-WDW resorts have an extra bathroom in the 1BRs) then you get two more bath towels in the second full bathroom.
I would hope that the resorts that have sleeping accommodations for five guests but no second bathroom (OKW 1BR, VWL studios, VGF 1BRs and studios) there would be five bath towels but I can't say for sure whether that is true.
Everything else has four bath towels in the 1BR.
If you need more towels, you can purchase a towel pack for $6.00. Since DVC is a timeshare, guests who need more of something (towels, shampoo, etc) are expected to pay for what they use.