SSI paid me $23755 but my net was $29148What do I pay tax on?

[gdd1]

They subtracted $5393 for           Medicare and drug premiums

[Bsch4477]

You pay tax on the whole amount. Although you didn’t receive the amount taken for medical insurance premiums you derived that benefit by having them paid for you. 
If you itemize those expenses will automatically be a medical expense deduction.