Low-pass elliptic filter design: equation for calculating required minimum order
The estimate is just that: an estimate. If the estimate yields $N=2.4$ then you're pretty safe with a $3^{\rm rd}$ order filter. If the estimate yields $N=2.95$, then a 3rd order might work, but there is a high likelihood that a $4^{\rm th}$ order is required. In cases where the estimate is just under a whole number, the way to be safe is to either increase the estimated order by 1, or to design both a $N$ and $N+1$ order filter to see if both meet your requirements.