Is there a maximum number of Grandmas you can have or any other item?
You can buy different sources to produce your cookies.
But is there any actual maximum in grandmas or factories you can have?
The game itself does not limit it in anyway, but javascript does. I'm not sure what happens after you get 2^53 of one factory as javascript loses its ability to keep track of individual integers. 2^53+1 = 2^53.
Another thing to note is that the cost to buy the next factory becomes Infinity relatively fast, which means you will need a supernatural cookies per second stat to make any progress in any reasonable timespan.