You are very young to be needing cortisone injections. Except in cases of significant inflammatory arthropathies, such as rheumatoid arthritis, the use of cortisone is not recommended in patients with fairly normal articular cartilage.
Though cortisone was used pretty willy-nilly in the past, it is now not recommended, except in cases where oral medicine has not been able to control severe intraarticular inflammation.
It has been shown that cortisone can damage normal cartilage, if used in high doses or multiple times.
So, if you do need the cortisone for a severe inflammatory process, then you need it.
But, no, you will not need crutches after having an injection of cortisone. You will be a little sore at the injection site, and the knee may feel a little full. But, if you have significant intraarticular inflammation causing severe pain, the cortisone will make the knee feel much better.