By Mars sized I mean with masses between 0.10 (M⊕) and 0.25 (M⊕), the planets need to have independent orbits around the star. We can assume the habitable zone stretches from 0.70 AU, to 1.5 AU for a typical G type star like the Sun. I don't know if adding a gas giant to the system will cause instabilities, but I think they are important for perturbing comets and other volatiles to terrestrial planets.
.7 to 1.5 A.U.?
Venus is .72 A.U., earth is 1 A.U. and Mars is 1.52 A.U. So judging by our solar system, 3 planets are possible.
Smaller bodies at a star planet L4 and L5 are stable if the smaller bodies are 1/25 the mass (or less) of the larger planet. And 0.04 (M⊕) lies below the 0.10 (M⊕) boundary you suggest. Unless the larger planet is 4 times the mass of the earth, then it could have a Mars sized trailing as well as a leading Trojan.
Would a planet 4 times earth's mass destabilize the other planets within the habitable Goldilocks zone? Sorry, I don't know. If a super earth is possible, I could see a super earth with two Mars sized trojans as well as two other Mars sized bodies in other star centered orbits. So my guess is up to 4 Mars sized bodies and a super earth are doable within a sun like star's Goldilocks zone.
Edit: JDługosz suggested double planets are possible. I believe this is true.
So I think 6 are possible. Here's a pic:
At 1.5 A.U. is a super earth with a Mars like moon. Also at 1.5 A.U. are two Mars sized Trojans.
At 1 A.U. is a single Mars sized body. I want to keep this guy small so he doesn't destabilize the super earth trojans.
At .7 A.U. is a double planetoid, each Mars sized. This might be far enough from the 1.5 A.U. orbit that a more massive double planetoid won't destabilize the trojans.
Speaking of Mars sized moons, that raises the possibility of one of my favorite settings: A Gas Giant In Earth Like Orbit (GIELO) and an Earth Like Moon (ELM). I talk about GIELO and ELM on my ZRVTO post.
Based on the series of essays Building the Ultimate Solar System, one of his “super” systems comes to mind.
3 stable orbits of different radii. Maybe need a wider spacing if we play more tricks, but you can push the far end of the zone by using greenhouse effects.
Each orbit might contain 2 bodies 60° apart, being in each other’s trojen points.
Or, each body might actually be a double planet! So that’s 6 to 12 bodies.
The presence of additional giant planets will mess that up. HopDavid’s variation uses a giant with a habitable moon to get 3 in one orbit, but that means the other orbits can’t share so much and I still worry about the eccentricity being pumped up. So ditch the giant completely and put 2 to 4 Mars bodies in each orbit.