If you choose the first mode for Glorfindel, Dauntless Rescuer's triggered ability but each creature the defending player controls can't block for any reason (such as being tapped), then Glorfindel, Dauntless Rescuer isn't blocked. If there's a cost associated with blocking Glorfindel, Dauntless Rescuer, the defending player isn't forced to pay that cost, so it doesn't have to be blocked in that case either.

If you choose the first mode for Glorfindel, Dauntless Rescuer's triggered ability, only one creature is required to block it. Other creatures may also block it (unless you've also chosen the second mode for Glorfindel, Dauntles Rescuer's triggered ability this turn) and are free to block other creatures or not block at all.