You don't need anabolics at all for your goal. You need to do a lot of pushups to failure. Lifting will help some, but mostly if you want to increase pushups you need to do a lot of pushups, end of story. If you aren't already try doing different variations too, like incline, decline, wide hands, close hands, tricep pushups(arms tucked into sides). Doing these exercises will help you build the muscles that your body uses to do push ups. If you can do 21 at the most, do sets of 21. Also make sure that you are getting adequate rest because if you are doing a hard workout targeting these same muscles each day then your muscles might not be recovering.
If you're deadset on anabolics, I hope you have a lot of lifting experience, especially as a female since anabolics can have a profound effect on you. According to the US NIH(National Institutes of Health) a healthy male's test levels will measure 300-1000 ng/dL(nanograms per deciliter), while a healthy female will measure 15-70 ng/dL of test. This means that in a female hormone enhancing products can have profound, and sometimes permanent effects like deepening of the voice, enlargement of the clitoris, and shrinking of the breasts to name a few.
That being said some women have taken low dose anavar or low dose winstrol without these problems occuring, though I'm not sure of what the actual dosing should be. DO NOT TAKE HALO FOR HER OR HALO EXTREME. I have been told these products have not worked well for women so I wouldn't risk the dangers.