Could anyone advice me on how many rooster to hens I should keep? I do have a few EE hens that I could bulk up the numbers with.
One rooster for 10 hens is often a good ratio, but it can vary from one breed to another and even from one rooster to another.
With just 5 hens, I would not want more than one rooster with them. One rooster with 5 hens is good with some roosters, but some other roosters overmate the hens with that ratio. That is something that you will have to try and see what happens with the rooster you choose to keep. Yes, adding some EE hens would probably help with that.
If you want to keep a second rooster as a spare in case anything happens to the main one, then you could either keep both roosters in one pen with all the Ayam Cemani hens and some other hens as well, or you could keep one Ayam Cemani rooster with the Ayam Cemani hens and put the other rooster with some other hens in a different pen.
Have notice 1x male has a cross beak but looks fully grown.
I would definitely not use that one for breeding.
For the others, examine each one carefully and see what good and bad points they are, then use that to decide which ones to keep for breeding. I do not know enough about Ayam Cemani to say what specific things to look for, but in any breed you want to check for general good health, along with the correct traits for their breed (body shape, feather color, number of toes, comb type, feathered vs. clean legs, etc.)