This may be happening on other consoles as well, but seems to not be an issue on the PC. My stardust armor pieces each say "increases your max number of minions by 1" giving the appearance that the full set gives +3 minions. But it actually gives +5 as the breastplate and leggings should say "increases your max number of minions by 2". To be clear wearing the full set does indeed increase the max number by +5 so it's just a tooltip error.