The method involves transmitting to receivers receiving a multicast a plurality of requests for feedback (3), each request including a probability parameter (P). Each terminal replies to this (or not) with a corresponding probability (4). One then counts the number (r) of replies to each request (5); determines, from the counts and parameters, estimates of the number of receivers (6); and filters the estimates (7). The method further includes repeatedly computing a new probability parameter to be included in a subsequent feedback request, by forecasting, from the counts and parameters, a upper bound for the number of receivers (9, 10, 11) and determining from this the new probability parameter (12) such that the risk that the number of replies exceeds a predefined threshold is kept below a predefined value.