It is very understandable to get concerned about number of lesions but resulting symptoms are not closely related to number. Location is more important. If there are more than a few the number is probably not worth worrying about. The MRI machine and the scans requested will influence the number detected. If there are more and diagnosis is clear cut what does having an exact number help?
If you are still concerned it is probably better to ask your consultant as the MS Nurse may not have the information anyway. You can request your MRI results from the hospital. The raw scans are interesting but in comprehensible to us, but the accompanying report is more useful. Someone many years ago used to help with interpreting these and a guide is available in the forum.