I can't imagine there is a Vegas line. I don't think Vegas is actually allowed to take wagers on non-sporting events.
As for how many, lets do some quick figuring.
- Declared or already have exploratory committee (I'm not counting crackpots and others who have no shot): Warren, Castro, Rep. John Delaney, Washington Gov. Jay Inslee (4)
- Major politicians who look really, really likely to run: Beto, Biden, Booker, Brown, Bloomberg (the five Bs), Harris, Sanders, Gillibrand, Klobuchar, Casey (10)
- Lesser known politicians who have expressed some interest in running: South Bend mayor Pete Buttigieg; Hawaii Rep. Tulsi Gabbard; LA Mayor Eric Garcetti; Colorado Gov. John Hickenlooper; fmr. Attorney Gen. Eric Holder; Va Gov. Terry McAuliffe; Ohio Rep. Tim Ryan; CA Rep. Eric Swalwell; losing FLA/GA governor candidates Stacy Abrams and Andrew Gillum; New Orleans mayor Mitch Landrieu (11)
- Celebs/Businessmen who look likely to run: Billionaire Tom Steyer; Fmr. Starbucks CEO Howard Schultz; Oprah; Angelina Jolie; George Clooney; the Rock; Facebook COO Sheryl Sandberg (7)
So, that is 32 folks and while some of them clearly won't get in (no way all those celebs get in the race) I strongly suspect we end up with something like 20 truly viable candidates. A lot of them will quickly realize they simply are not getting any traction (and any money) and drop out shortly after coming into the race, but it is going to be a very crowded field into at least the fall.
-Jason "I am sure I missed a couple... and I bet there will be a surprise or two as well" Evans