I'm told that using count till a hundred is improper grammar. What's the correct preposition?
till is an informal variant of until. Oxford Dictionaries says:
Less formal way of saying until.
They both refer to ending at a particular time or event, not a position. So you could say
Count till you get to 100.
because you get to 100 is an event. But you wouldn't say
Count till 100.
because 100 is a number, not a time or event. In that case you would say
Count to 100.
I was taught to use: 'Count to 10, 20 etc'.
Here is also an example
'Count to' means 'Recite numbers in ascending order'