As far as I can see these are the rules:
From here: https://www.premierleague.com/news/464747- Premier League clubs may not register more than two players on loan at any one time.- The maximum number of loans registrable in the same season is four, and, under no circumstances, shall more than one be from the same club at any one time.- Premier League clubs cannot loan to another Premier League club a player they have acquired in the same transfer window.- A Premier League club may loan not more than one of its goalkeepers to another Premier League club.
So that reads as you can have four in total but only two at one time, so I guess that means you can have two in the first half of the season, they get recalled or club sends them back in Jan and then two more loans come in for the remainder of the season.