Based on the information you provided, the heroine traveled to a world full of beasts and met several characters.

The specific number of male protagonists, and how many of them are mermaids, may need to be determined based on specific circumstances.

Here are some possible plot developments: 1. The heroine encounters several beastly male protagonists, including a mermaid.

These male protagonists may be important roles for the heroine in the beast world, and they may provide help and support during the heroine's adventure.

2. The heroine meets a beast hero and one or more other types of heroes, such as wizards, warriors, etc.

These characters may play an important role in the heroine's adventure, and they may provide help and support when the heroine needs help.

In short, the heroine meets several important characters during her adventure in the beast world. These characters may be the male protagonist, and some of them are mermaids.

Specific storylines and character relationships need to be determined on a case-by-case basis.