As previously mentioned, the number of keys will depend on your child’s age and skill level. Beginners who are interested in learning the instrument should have at least 49 keys so that it accommodates two-handed play. However, toddlers can get away with fewer keys until they’re old enough to really focus.
A standard keyboard piano has 88 keys and there are plenty of options in between these two sizes. If you want a keyboard piano that’ll last, the bigger the better so there’s room to grow.