Bourne, If you are talking about the segment limited RTW fare (as opposed to mileage limited one) there is NO limit on mileage. In fact, if you had your ticket under the old rules, you could buy unlimited additional segments and have very high miles (could even go up to millions though they may balk at it as an abuse of rules).

Under the current rules, you are limited to a max of 8 segments in NA and 6 in other continents. There are limits on long segments (only one NA transcon non-stop or direct and only 2 to MiddleEast and select long haul destination in Europe, no backtracknig from Hawaii, etc.).

I have calculated over 50,000 base miles (and multiply it by your status/class of service bonuses) on many simple 3 continent trips using free segments. You can add another 25K to 50K based on additional continents/segments or using complicated runs.