"Benign tumours" casts a very wide net and some firm definitions would be needed to delve into the specifics of the majority. I presume this question is being asked as you or a family member has multiple lipomata.

Lipomata are not fully understood - we interpret them as predominantly benign adipose tumours that occur in many populations, not just humans. Contemporary thought suggests those with the Murine Double Minute 2 gene are more "atypical" and thus have a greater chance to grow aggressively (locally) relative to a standard lipomatous lesion. This does not necessarily mean they are malignant though. Of course they may end up being liposarcomas, but these are super rare. So all in all, the more lipomas an individual has you could suggest they are more likely to have one that is aggressive and possibly malignant purely based on the sheer number. The rate is negligible though and on a practical basis you're more likely to get another harmful cell type malignancy. As others have pointed out there is not much point researching further into lipomas because the morbidity and mortality is tiny.

Other lesions can be associated with increased risk of malignancy, however; epidermoid cysts are very common and are never aggressive; although, for instance, a lot of them being present can be sequelae to Familial Adenomatous Polyposis (FAP) aka Gardner Syndrome in which polyps grow and almost always progress to malignancy unless treated. Although you would suppose that is not the lesion itself being cancerous, just associated.

Much like all of medical science, there aren't black and white rules to lumps and bumps; just trends, relationships, and probabilities. The likelihood of a lipoma turning nasty is very very low (e.g. NHS sarcoma service data suggests the UK gets <1000 full blown liposarcoma diagnoses in total in a year, when you consider how many lipomas there are around the ratio is absurd).

Edit: https://pubmed.ncbi.nlm.nih.gov/25308007/ https://www.webpathology.com/image.asp?case=665&n=7#:~:text=MDM2%20stands%20for%20Murine%20Double,%25%20to%2037%25%20of%20sarcomas.