This is the current order of the series as of now, and the modern platforms they're available on:

Trails in the Sky FC -- (PC)

Trails in the Sky SC -- (PC)

Trails in the Sky the 3rd -- (PC)

Trails from Zero -- (PC/PS4/Switch)

Trails to Azure -- (PC/PS4/Switch)

Trails of Cold Steel I -- (PC/PS4)

Trails of Cold Steel II -- (PC/PS4)

Trails of Cold Steel III -- (PC/PS4/Switch/PS5)

Trails of Cold Steel IV -- (PC/PS4/Switch/PS5)

Trails into Reverie -- (PC/PS4/Switch/PS5)

The 11th game in the series, Trails through Daybreak I, will be releasing in July on PC/PS4/Switch/PS5 I believe.