The maximum number of minterms that the SOP form of $F$ can have such that no simplification is possible (i.e. Canonical SOP form itself is the minimized SOP form) is $8.$
The following set of True Minterms for function F will lead to No simplification of Canonical SOP possible.
$0,3,5,6,9,10,12,15$
Note that F' also cannot be simplified.
$\mathrm{F}^{\prime}=\mathrm{X} \oplus \mathrm{Y} \oplus \mathrm{Z} \oplus \mathrm{T}$