Okay so you want to buy a keyboard with your piano but now you have decisions to make. Based on your budget and experience you need to figure out how many keys you need.
Much of this decision making is based on personal preference and of course what your budget is like. Of course if you are a perfectionist money can be no object. Unless you are working 2 jobs to cover the cost of the keyboard that you want you would be able to afford it.
In any case the keyboard has got to feel right for you. Cost may come into the decision making process if the price is exorbitant but you must feel good with the keys so that you can come out with the best sounds you can muster.
To get the most affordable keyboard with the most keys you’ll probably go searching at used keyboards and that is where you have to know how old the keyboard is. Because the older a keyboard then the tendency to decay becomes more and more. This decay affects the sound and the wear and tear of the keys themselves. Make sure none of the keys stick together or that 2 keys end up being depressed when you hit just one key.
Next in line for the decision is how many keys do you want to have on the keyboard? Personal preference wins out again but for the best sounds you should go with the full gamut of 88 keys. If you’re just in training mode and just want to hear music anyway you can then you can opt for the lesser amount of keys at 61 or 76 keys. 61 keys will suffice but soon enough you’ll want better sounds from your efforts at the piano.
But for your money and time spent at the piano going for the 88 keyboard is your best bet.