How many layers of singles can i put on my roof?

my roof on my large house that was built back in the 50s or 60s not sure. and it has add ons that were built in the 80s and 90s well i just don't make allot of money and don't want to go into any more dept so when the old part of the house started leaking i just put roll roofing on it and did a very good job using roll roofing sealed the roof up not one leek its not my skill in building that worries me its just that i don't have time, when i added the roll roofing i think it only had 1 layer of shingles so now i think it has 1 layer of shingles and 1 layer of roll roofing, now if i add anotherr layer of shingles the total that will be on the roof is 2 + the layer of roll roofing, is this to much, don'tnt have time to tear off the old roof now, but i now i can put more shingle over the roll roofing to make it look lot better, and if it last another 10 years all be ahead, i plan on staying in this house a long time. the old house was built pretty good but don'tnt want to add to much weight to her, a couple of people have told me because of the extra weight i should not add more shingles but these people have 0 building experiencece, now i have building experence but i am no enganeer, i just dont want to get some crasy surprise where i add anouther layer of shingles and the whole house falls down , witch i dont think is going to happen, please give me some advise, it would help me out lots if i could just roof over this old roof, how can i get away with this, with out causing any damage to the house, i no if i was a ritch man i would strip the roof down and do all kinds of things but i am not, and i would rather have a old house with a old looking roof than no house because i went into to much dept. thanks