Here is a piece of advice that i WISH someone gave me....
my fiancee and i had to whittle our lists down just so all of our guests could fit into our dream venue. and now. 3 days away from our RSVP date, we are getting single people sending in repsonses with +1's, when we had to cut out other friends just to fit the friends we did invite, so there is no way we are including dates. we told as many of our non-married/single friends as we could, but they just don't get it.
so... what i would tell any brides to avoid this for their wedding, is to send all of your single/non-married friends a text or email or possibly a note along with the save the date, saying that you have no room for additional guests, since you are at max capacity for your venue and that although you would love to have them at your wedding, you cannot allow them to bring a date. that way, they know from the get-go, and if it really bothers them that much... they have the option of declining your invitation.
i really wish someone told me this in advance, then i wouldn't have had to learn the hard way and had to have so many awkward conversations with friends... a few days away from when our final numbers are due