The former Bristol City manager will be recruiting new faces to add to his promotion-winning squad with the goal to have between 18 and 20 players fighting for places.
He said: "It is nice that we have the crux of our team still with us, it does make it easier but you do have to add to it.
"You can't stand still at any level. We can't stand still and there will still be competition."
Last season's spine included the likes of Marek Stech, Paddy Madden, Ed Upson, Jamie McAllister and Luke Ayling who are contracted for the forthcoming season.
And having committed his future to the Huish Park club Johnson believes none of his squad are looking to leave Somerset ahead of the new campaign.
"They all seem happy, they all had a great time and from the comments I hear from them they have had a wonderful last month or so," he said.
"So I am sure they'll be happy to be around this club for a little while longer I hope.
"If players are comfortable where they are then they perform to the best of their ability and that is what they are going to have to do in the Championship."