The most important consideration is the number of mounting bolts and the bolt circle diameter (BCD). This determines what will fit and also the smallest possible chainring.

Two common BCDs are 130mm, typically used for road, and 110mm typically for mtn bikes.

I suspect that you have a 5 bolt road crank with a 130mm BCD, in which case the smallest possible ring is 38t.**

Confirm by measuring (on center) between two adjacent bolt holes and multiplying by 1.7 (round up).

** Note----- 38t 130mm chainrings tend to be pricey, 39t is the traditional standard, readily available at low cost.