Some things to consider are: indoor or outdoor ceremony, budget, space available to place musicians and the kind of music they wish to hear.
If your wedding is indoors and the venue will have nice acoustics, you may not need a large ensemble and may get by with just a diet or solo. Outdoor weddings need more musician power to achieve the desired effect and one instrument may not meet your expectations. Therefore, you should think about hiring two or more musicians to provide more sound to fill the space.
Budget is perhaps the biggest dilemma for brides to deal with and it will dictate the size of your music ensemble in a very direct way: go with you can realistically afford. Here is your chance to set your priorities. If you would rather have rare flowers, or more expensive dress, you need to remember that music is also an important component to a memorable wedding day. Music should not be an afterthought. It's the first thing your guests will hear and it will punctuate the emotional highlights of your wedding ceremony. Therefore, it should be built into your wedding budget from the beginning. I've seen brides who could have bought a new car for what they paid for their wedding dress, and because of the choices they've made, they had to deal with a greatly reduced music ceremony budget, hiring one musician instead of four.
My advice is to hire a string quartet if your budget allows it. It's a Lexus of ceremony music compared to one musician, which would compare to a Ford Focus in this context. It is up to you which option will best serve your event. After all, it's going to be in your memories, and only you can put a price tag on that.