There are 3 features that are important :

1. That the keys are standard size (not small eg childrens size)

2. That the keyboard has at least 61 keys (counting both black and white keys)

3. That it is touch sensitive (ie plays louder when keys are pressed harder)  

We have a few students who are using keyboards that have less than 61 keys and/or a keyboard which is not touch sensitive and you can definitely start making progress on the course with this type of keyboard and then upgrade to the recommended type of keyboard whenever you get the chance.