Start with the amount recommended for your particular warmer. One or two cubes may be enough for some setups, but there is no universal cube count: a large snap-bar segment can contain much more wax than a small cube from another brand.
Check the warmer's capacity before adding wax. The portion needs to fit comfortably once melted, with space left below the rim. A large dish is not an instruction to fill it, and a small warmer may not be intended to fragrance an entire open floor plan.
Let the first portion warm before deciding whether to add more. If the scent is already comfortable from where you spend time in the room, you have enough. If it seems faint, step out briefly and return before assuming the portion is too small.
An extra cube adds wax and fragrance, but it does not guarantee a proportional increase in strength or duration. The warmer's heat and the resulting wax pool affect how the fragrance is released. More wax is useful only if it improves the experience in your setup.
If the warmer has room and its directions allow it, try a slightly larger portion on a later session using the same scent. Changing the amount, scent, and warmer all at once makes it difficult to tell what helped. Once you find a portion you like, use that as your starting point for similar melts.