I am a property manager and have been for 15 years. Your need is going to be based on your skills and your personality. There is a huge difference between a good property manager and an ok one, and even more difference in a bad one.
If you are the type that likes to research a subject and go all in then you wont need a manager, until you have enough doors that you have to start watching the changes in Fair Housing Laws and State and County, and City Laws. If you don't mind handling calls for property questions after hours, or going after rent, or time in small claims court, that you really don't need one. If you like to do things your way and you would rather hire a handyman to run repairs after you have inspected, rather than delegating these duties, a property management company will be a hard fit.
If you prefer to manage your portfolio and keep an eye on markets and your ratios for each rental, then it is time to have a professional handle the day to day items. A professional manager will:
1) Limit the risk of lawsuits and risk on tenant safety
2) Perform home turns between tenants quicker than you can
3) Screen tenants with advanced knowledge on Fair Housing Laws, identity theft, and filtering false references
4) Be able to monitor and handle scams
5) Provide you with the quickest eviction process if needed
6) Have problem solving skills, so that most days you don't even know there was an issue because it resolved with little or no involvement from you.
7) Have resources that include eviction attorney, repairs, cleaners, appliance repair, screening companies, mortgage properties, emergency line coverage. Ideally if you go the professional route, you need to go with a manager that knows the area, and how to deal with all types of people. The other mistake I see individuals make when hiring a manager is they hire a Realtor that doesn't really do management, but will as a side hustle. You want to have the managers focus even when sales are good in an area.
Once you own more than 4 doors in a community or 2 in states you don't live in, it is worth it to have a professional, as that is when new legislation in areas will have a higher risk for you as a landlord.
If you are a personality that does your own business, and private taxes, repairs your own car, manages your own rental flips and finances properties on your own, then you more than likely don't need a management company.