Assuming that you have a field called CustomerID that identifies the purchaser then something like this should work:

int customerIDValue = theCustomerIDYouWantToUse;
OleDbCommand oledb = new OleDbCommand();
OleDbConnection connection = new OleDbConnection( "YourConnectionString" );
oledb.CommandType = CommandType.TableDirect;
oledb.CommandText = "SELECT Count(*) AS CustomerPurchaseCount FROM purchase_table Where CustomerID=@CustomerID";
OleDbParameter sp1 = oledb.Parameters.Add( new OleDbParameter("@CustomerID",OleDbType.Integer,4   ));
sp1.Value = customerIDValue;
connection.Open();
int count = (int)oledb.ExecuteScalar();
connection.Close();