I don’t know where this etiquette came from, but the bridesmaids ratio is suggested to be 1 bridesmaid for every 50 guests you have. I’m not a stickler for etiquette but it gives you a guideline. Because if you have a wedding of 50 guests, you don’t want to go with 10 bridesmaids. You might as well invite everyone there up with you!
My wedding was 230 guests, but I had only 2 bridesmaids because I kept to close family members: my sister and my cousin. I couldn’t decide among friends and I was too afraid to offend people because if I asked one person, then I felt obligated to ask 3-4 other people who were in the same group of friends. So I made it less complicated and in the end, it worked out because with our short timeline to plan the wedding, it was less to coordinate.