The number of ridge unions required is not the same as the number of ridges: for instance if you were using 50 No. ridges to fit along one ridge length, there would only be a need for unions at the point where two ridges meet, in this case 48 No. unions would be required, plus 2 No. Ridge End Caps or Block End ridges, if it is a gable to gable roof. .
It is also worth noting that the ridge Unions and End Caps or Block End ridges constitute a mechanical fix, due to BS 5534 specifying that each ridge tile requires two mechanical fixings.