Near the start of the respective descr_strat files there's a value for brigand_spawn and pirate_spawn. In BI they are 10 and 12 respectively and in RTW they are 10 and 28.

Essentially these numerical values are divisors and therefore, the lower the number the more rebels/pirates will spawn, the higher the number the less likely they'll spawn. For example, a value of 20 for brigand_spawn will reduce the likelihood of rebels spawning by a half.