Doing more reps is not going to help you build more muscle. You are strong enough now that your own bodyweight is not challenged by doing push ups. You need to find mroe chest exercises to do with heavier weights.

When you work out doing only 8-12 reps before you give out you are causing the muscles in the chest to tear slightly. It is when these tiny tears in the muscle fiber that when healed by the body build more muscle. At 50 push ups you are only building muscle endurance not mass.

I would like some of you advice too. I am 15, i do planks, pushup (5 per workout and increased per day, now near 50). I eat eggs everyday, especially half fried and boiled. i also eat basic bodybuilder’s food like fruits , chicken and lots of beef. My stomach is quite lean but i have no chest muscle. i really need to fit up with me stomach.

Can you help me?

One important thing to note is that stronger arms should come from your triceps not your biceps. Triceps (under the arm compared to the bicep) are the working muscle for most stuff you do, biceps are more a show muscle if they are huge. Lots of guys work their biceps too much when the triceps will give you more strength and still look great when they gain a lot of muscle.

Start with push ups and pull ups if you can, but if they is too easy (if you can do more than 12 reps with ease for instance) then you will need to add some resistance. There are so many arm exercises it is hard to give a full list but work the triceps, biceps, shoulders, and chest – or workouts that combine multiple ones of these.
Forearms and wrist exercises are good too but I admit i neglect them in my workouts.
Last bit of advice is to not just concentrate on the arms even after all that stuff I just said. Makes sure you work your whole body – but if the arms are an issue lean a little more to the arms and chest when working out but do not neglect your core, legs, and back too.