Defining number of UEs constant per ROI makes the number of UEs independent of the number of femtocells.
I suggest you have a look at the functions that place UEs in your scenario. You can find them in the folder +spatial_distributions. You might have to build your own function for for UE generation and include it in LTE_init_generate_users_and_add_schedulers as option for UE_distribution.
Please let me know if you have any further questions regarding this issue.