How to retrieve data by user_id Or any suggestions are welcome

I currently have a php webpage that people register and log into. Now I need to let them enter data into a form. Then store it into a table. But here is my dilema…I will have many users inputting data for their company. Once they input their client information I need a way for them to choose what client’s information they would like to view and make sure they can only access their clients information and not a client’s information from another company. From what I am told it is not good practice to create multiple tables?? So I am not sure how to go about this. I need to make sure they can only access their client’s information.
Does anyone have good ideas how I can structure this?

You can use multiple tables if you want?

Client_id > user_id

or store a user_id and multiple client_ids

so…

user_id = 1
client_ids = 1,3,4,5

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