There is no specific number that can be applied to all types of communities. It depends on the purpose of building the community, number of the initial contents you want to post, and number of the community members. However, we suggest 5-10 bot users for every 10 content. So if you are looking to seed 100 content, you are looking at 10-20 users.
The purpose of using bot users is to build an engaging environment and to encourage other community members to interact with each other. Community admins/moderators can use the face of bot accounts for posting content and up-voting others' answers. This way, your community looks more crowded and active in the early stage, and your new members feel there are others in the community too.