ERAS charges the same fee for applying to up to 10 programs, so there's really no reason to do less than that. Keep in mind that you will most likely be applying to medicine prelim/transitional year programs as well, as many programs either do not offer a categorical program or have both categorical and advanced positions in the match.
I'm curious as to the reasons for restricting your scope of applying. Cost isn't that prohibitive, and you have plenty of ability to be selective once the interview invites start coming in. With the match becoming a tighter process each year, you are doing yourself a huge disservice to not maximize your chances in the application phase, where the cost is very low compared to the total amount you will spend during the interviewing process. Not to mention the fact that your ability to judge a program and find one that is right for you increases with your exposure to other programs during the interview season.
Ideally, you will at the very least want a good mix of programs at, above and below your perceived level of competitiveness. You should also want to apply to a good amount of prelim and transitional programs, as those spots can be more competitive than neuro spots at the same institution. I personally applied fairly broadly and am very pleased with the result of my match.