You might want to ask that question on the Boatnerd forum. Finding the exact number of steamboats for the 1840-50 time frame will be hard; the Steamboat Inspection Service was formally established in the Treasury Department in 1852. The Minnesota Historical Society had this information on their website: "By 1840, there were more than 100 steamers in service on the Lakes. Most were less than eight years old. About 40 of these craft operated as ferries or on short local routes out of the larger ports. The remainder, principally the larger boats, ran from Buffalo to upper Lakes ports or from Niagara and Toronto to lower Lakes or St. Lawrence River destinations."
Here is some information on steamboats on Lake Erie, although it is data from 1835.