I appreciate your concern over the lesions as you have so many of them. After seeing the pictures you have uploaded I’ll try and explain things to you in light of your queries.
On an average an adult male has about 10-20 normal moles (melanocytic nevi), which are round or oval (as your larger mole), have symmetry (if you draw a line dividing in half—you will get near perfect halves—as your large lesion is more or less), with almost regular margin (as you have—it may not be a perfect round or oval shape as you draw with pencil but more or less round or oval). They may vary in size, with the largest being about 6mm in diameter. So, to answer your first question, not all nevi more than 5 mm in diameter are dysplastic nevi.
Moles are often present in the sun exposed areas: face, upper arms, forearms, upper back etc. So, that is why you have most of them on arms. They have symmetrical color (as you have) and they resemble each other (quite likely in your case—need a closer examination of all nevi to comment on this aspect).
Both dysplastic nevus and melanoma have several common characteristic such as asymmetry, irregular border, color variation and diameter of 5mm or more. However, a dysplastic nevus has general color variation with a central darker portion and an outer lighter portion or a central papule with outer colored ring. Also the borders are hazy and merge with the normal skin. If it undergoes a melanoma change, the color distribution within the nevus becomes irregular.
To have atypical mole syndrome, you must have two or more atypical moles (larger than 5 mm, with irregular margin and color variation, and oval shape). You do not appear to have these as per your description. So, based entirely on your description of all the moles you have, you do not appear to have atypical mole syndrome.
The risk of melanoma increases with a family history of melanoma, family history of atypical mole syndrome, and having more than one or two dysplastic nevi. Since you do not have a positive family history, in your case, the more the number of dysplatic nevi the higher would be the risk. However, having a large number of moles whether normal or dysplastic increases the risk of melanoma compared to another person who has few moles.
Moles and freckles look almost the same. Freckles are just flat pigmented areas with no cells. Moles have cells and can be flat or raised. On naked eyes, it is difficult to differentiate between the two. However, dermascopy will be able to differentiae the two. If you observe them, freckles change in size and color with exposure to sun and new ones will keep appearing with increased sun exposure. However, it is not advisable to purposefully expose yourself to sun for long periods to test this.
While the larger and smaller lesions in your case appear to be moles, a more confirmed diagnosis can be made on closer clinical inspection and possibly dermascopy to visualize the lesions under microscope. Also, since you have such a huge number, it is advisable to consult a skin specialist regarding this.
I sincerely hope you will find this information useful. Good Luck and take care!
Thank you so much for your help. Just a bit more to ask if you could give me a few quick answers.
1. My father (who is 62) also has a lot of moles, and sees a dermatologist once a year. The dermatologist has had a few small red bumps removed from his face, but they were not moles and were not cancerous - he just wanted to do it as a precaution. Since then everything has been fine. The dermatologist never mentioned that he may have any genetic predisposition towards melanoma. Given that FAMMM/DNS/AMS have very serious implications for the future melanoma risk, is it a good bet that if the dermatologist didn't tell him he had it that he likely did not have it? Would a dermatologist ever withhold that information for any reason?
2. If neither my mother or father have DNS/FAMMM, is it a safe bet I do not too?
3. Do those with a genetic predisposition for melanoma typically start getting melanoma earlier? How early would you expect to see moles needing to be removed, or melanoma diagnosis in those with DNS or FAMMM?
4. Can simply having many normal moles run in families?
5. Can a normal mole grow into a dysplastic nevus, or do dysplastic nevi develop from normal skin?
6. If my only risk risk factor is larger than average number of moles, how much more at risk would I be?
Thank you so much - these will be the last questions I'll ask. You have been very helpful.