I thought it'd been changed to 18 now?
Unless it's just been changed, 8 players must be home-grown, and 4 of those must be Liverpool-trained (3 seasons training in the country and/or at the club before turning 21). That leaves 17 of the 25-man squad who don't have to fit that requirement.
Current U-21 players don't need to count against either restriction, but if a club registers fewer than 4/8 home/Liverpool-grown, than you lose the corresponding number of roster spots from your 25, so an under-21 player might be included in the 25.
So for us (and I'm spitballing here), our English home grown players are Gomez, Henderson, Milner, Oxlade-Chamberlin, Trent, Curtis Jones, Harvey Elliott, Rhian Brewster and Nat Phillips. I know Scots players do not count for this but I think Welsh players like Neco Williams (under 21) and Harry Wilson (over 21) might (anyone?)
Players meeting the Liverpool-trained requirement are Gomez, Trent, Curtis, Rhian and Nat (Harvey Elliott has only been with the club one season even though his age is right). We will need four from that list on the squad list, or we would not be able to list 25 overall.
So against 17 non-homegrown players (over 21) we currently have 15:
(I'm not counting Loris Karius as I can't imagine he will be in our registered squad. Caiomin Kelleher turns 22 in November, but I'm not sure when the cutoff date is. He would count as home-grown since he's been here since 2015.)
So if we register 8 English and 4 Liverpool-trained players from those above lists, even if we keep all 15 of the foreign players listed, you could still bring in and register two more foreign players.
Since the transfer window closes in October this season, I presume that is when the squad registration deadline is.