Black Sea fortnight, Sofia city break or a week in Bansko
In summer most visitors fly straight to the coast, into Varna or Burgas, for a week or two around Sunny Beach, Nesebar or Sozopol. For a fortnight of maps, messaging and beach photos, the 5 GB for 30 days is the comfortable size; 3 GB covers a lighter week. A city break in Sofia with a day trip to the Rila Monastery or Plovdiv fits in 3 GB, and in winter the same pack covers a ski week in Bansko or Borovets. Driving across the country, from Sofia through Plovdiv and Veliko Tarnovo to the coast, keeps navigation running all day: take 10 GB.
The 20 GB is for a month based in Sofia or Plovdiv, working from a laptop over the phone's hotspot. Whichever you choose, the days start at the first connection in Bulgaria, so you can buy and install it weeks before the flight.