How many koi to keep and kind of netting to prevent it from jumping out

How many Kois can I keep in a 6’x3’x1.5' FGT? Understand that a nylon net over the FGT is necessary to prevent kois from jumping out especially for new kois. Please suggest me a suitable size of the “eye” of the net so that it could prevent kois from jumping out and also not affecting too much viewing. I saw along Jalan Besar hardware shop the netting eye is range between 1cm sq to 3 cm sq; and also different type of net thickness are available. Can I use a thin net so that viewing is least affected? Thanks!