I work for multiple marketing companies and have many business expenses. Do I have to fill out one T777 with all my expenses or a T2125 for each company

T777 is for employment expenses, meaning that you are a salaried employee, and not self-employed.  If this is the case, your employer must sign a T2200 Conditions of employment form, then you would claim the expenses on the T777. You would need a T2200 and T777 for each employer

If you are self-employed, and paid as a contractor for your work by several different companies, or clients, you would fill out one T2125 for all of your revenue and expenses.