6-sticker sheets have started appearing. Martin Bennett has provided scans of the first one. He found a pile of these, all the same, at his local Co-op.
I have to thank Roverman for letting me know a sheet of six stickers is included with the current issue of FourFourTwo magazine (No. 309) that was published this week.
UPDATE (24-01-2020 23:25): There are stickers in tomorrow's issue of The Sun along with a voucher for a free album, more information some time on Saturday.
UPDATE (25-01-2020 13:27): Scans of the sheet available with today's issue of The Sun included, thanks to everyone who e-mailed me. Another sheet will be included with The Sun on Sunday tomorrow.
UPDATE (26-01-2020 21:19): Stu has confirmed that the sheet Martin found in his local Co-op is the one that's available from The Sun on Sunday via a voucher. I've shuffled the listing to get the sheets in the right order.
Football 2020 - Sheet 1 - FourFourTwo magazine
296. James Milner (Liverpool)
236. Fabian Delph (Everton)
508. Nathan Redmond (Southampton)
275. Jamie Vardy (Leicester City)
560. Tom Cleverley (Watford)
129. Glenn Murray (Brighton & Hove Albion)
Football 2020 - Sheet 2 - The Sun, 25 January, 2020
180. Ross Barkley (Chelsea)
351. John Stones (Manchester City)
609. Rui Patrício (Wolverhampton Wanderers)
212. Andros Townsend (Crystal Palace)
484. Lys Mousset (Sheffield United)
409. Fabian Schär (Newcastle United)
Football 2020 - Sheet 3 - The Sun on Sunday, 26 January, 2020
541. Tanguy Ndombélé (Tottenham Hotspur)
382. Harry Maguire (Manchester United)
93. John McGinn (Aston Villa)
447. Todd Cantwell (Norwich City)
599. Andriy Yarmolenko (West Ham United)
144. James Tarkowski (Burnley)