Words like ‘once’ and ‘twice’ can be predeterminers as multipliers. For example:
“Once every two weeks” “Twice a day”
However, ‘once’ is very often a subordinating conjunction or general adverb.
1 ONCE (CS@_RR) 2000496
2 ONCE (RR) 1897790
3 ONCE (RR_CS@) 994010
4 ONCE (RR21) 550086
5 ONCE (RR22) 257464
6 ONCE (RR33) 55643
7 ONCE (NN1) 46754
8 ONCE (RR41) 31200