When figuring the loss of an external antenna connection, every connection should be counted as a 0.5dB loss in the RF link budget. This would include lightning arrestors, N or RP-SMA adapters, amplifiers, and barrel connectors. A lightning arrestor or amplifier may provide a more specific loss value in its accompanying documentation. In this case, use the value provided by the manufacturer.
The proper determination of RF link budget in external antenna installations is critical when calculating free-space loss and predicted coverage area. Always consult the manufacturer or installation professional should you have specific questions.