Question: Calculate number of equations and number of variables

I have 2 equations... f[1] := x[2]+x[3]+x[5]+x[6]=16; and f[2] := x[1]+x[2]-x[3]+x[4]+3*x[6]+x[7]=28; 


Is there a method that will allow me to count the number of equations?

Is there a method to display the indeterminants or variables which are in the equations i.e x[1],x[2],x[3],x[4],x[5],x[6],x[7]?

Is there a method to find the maximum indeterminant or variable from both the equations i.e x[7]?


Thankyou very much for any help with any of these questions 

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