Plan on a gap of about 3 to 8 weeks between sessions, and let your provider pick the exact number inside that window.
Two published sources give numbers you can hold a plan against.
Two sources, two ranges
Cleveland Clinic says most people need multiple microneedling treatments, usually about 3 weeks to 8 weeks apart. The AAD gives a narrower example for scars: if your dermatologist is treating acne scars, you may need three to five treatments, which are performed every two to four weeks.
Read the two together. The wider range is the general one, and the narrower range is an example tied to acne scars. They overlap but they are not identical, so ask which one fits the reason you are being treated.
Papers on microneedling in other settings
Microneedling also shows up in research on other problems. One NCBI PMC paper compares PRP with PRP microneedling in androgenetic alopecia, and another looks at radiofrequency microneedling with a thulium laser versus radiofrequency microneedling alone for photoaged skin. For a wider view of resurfacing options, see the ASPS article on skin resurfacing techniques for facial rejuvenation.
Fixing the gap between your visits
Ask your provider how many weeks they plan between your sessions and why. Then schedule every visit around that answer so the gaps stay where you agreed.