If applying under the Express Entry system, your English language ability will get you marks at two separate stages of the immigration process.
Your first official language ability can get you a maximum of 24 points on the 67-point eligibility calculation. A CLB 9 and above is worth 6 points each on the four abilities assessed, amounting to 24 points altogether.
When your profile is in the Express Entry pool, your first language ability is worth a maximum of CRS 136 points (when applying without a spouse/partner).
The specific IELTS score you should aim for will be as per the pathway applying under. The higher your score in IELTS, the more points you can hope to get as Canada immigration points.