I'd say two, but It could easily be seen both ways. Especally in poetry, just use whatever sounds best.
Say you were doing a say, haiku (5-7-5) then go with whatever fits like if the top line were
Top:
"Surely, I'm tired"
Second:
"However, I may not rest."
Thrid:
"Trial by fire"
Full:
Surely, I'm tired.
However, I may not rest.
Trial by fire.
- Pharus
"Fire" would sound better there with two syllables because it's first line counterpart "Tired" has two. When if the first line ended in one syllable, it would sound better to end with a one syllable "fire".