Several triallists will be given the chance to stake their claim to bolster Pompey’s central midfield options.
Frenchman Romain Padovani and the versatile Danny East have arrived this summer to strengthen the middle of the park.
But Blues boss Guy Whittingham is still seeking another signing in that role having lost last season’s regular Therry Racon.
Liam Walker has been invited back in a bid to win a contract but there will be others also present in pre-season, which starts on June 27.
And Whittingham is hoping somebody will prove their worth and convince him to sign them up.
He said: ‘I think 18 players is the minimum we need, that is the first thing we have got to bear in mind.
‘To be a useful first-team squad within this league you have to have 18 minimum and you have also got those five development group guys.
‘We have a couple of spaces left, so we’ll have one or two triallists to see what they are like.
‘There will not be a great deal of them, we are not going to repeat the situation we had at the end of January when there were all sorts coming in.
‘One or two central midfield players will be in the bunch we are bringing in on trial.
‘These will be on different levels, young players released from higher divisions and older players who have been from lower-league sides. A mix really.
‘They will not necessarily turn up on the first day of training, we will sit down and look at that this week or next week and decide when to bring these guys in.’
Whittingham’s other central midfield options include moving Jed Wallace inside from the right.
Skipper Johnny Ertl is certain to be one of those starting the season there.