Pompey are looking for up to four 'quality' signings to boost their existing squad.
But chief executive Mark Catlin highlighted there is leeway for that number to rise if a key member of Kenny Jackett's squad departs.
The Blues have yet to make their first summer recruit, as work continues to go on behind the scenes over potential deals.
Jackett has stated he's after at least one new central defender, a striker and a player to operate behind the front man.
A new right-back will be required if Nathan Thompson doesn't return next season, while four loanees saw their stays end.
Those temporary spots in the squad can also be replaced.
Catlin said: ‘If you look a those starting in terms of the loans (last season) it was one or two. The player who was a part of it and not re-signed is Nathan Thompson.
‘We've got two left-backs, and four centre-halves of which one is injured and one developing. Kenny has said it's an area we're looking at.
‘When you go through our squad we have a squad of 20-plus contracted next season. We don't want an excessive squad, but I stress resources are there.
‘We're looking for a mix of younger players we can develop aligned with two to four quality players in the areas Kenny has identified he wants to strengthen.’
Catlin knows the dynamics of Pompey’s recruitment will change if Matt Clarke or Jamal Lowe were to depart.
He added: ‘The other glaring flexibility is if a player leaves that is good for Portsmouth Football Club. We've got an agreed structure on money coming in. There's flexibility there.’