When i exchange trophies to StrengthCraft do i need to exchange all of them or only the one that can give me a total of 7,

i.e. i have a 5 Str, 2 Str and 1 Str Trophies, do i nee to exchange them all or can i keep the 1 for later use?

You can keep the Str 1 trophy for later. Sometimes I've had to wait quite a while to get the most out of my trophies. I think the best example was the time when the Gladiator traded in 22 Craft points (6+6+5+5), because this was the combo that resulted in least loss of extras.