Start with 1, apply 1->12, 21->21, 22->21, 2->2 (for final 2); a(n) = length of n-th term.

Terms

a(0) =1a(1) =2a(2) =3a(3) =4a(4) =6a(5) =9a(6) =12a(7) =16a(8) =22a(9) =31a(10) =42a(11) =56a(12) =76a(13) =105a(14) =142a(15) =190a(16) =258a(17) =353a(18) =476a(19) =640a(20) =870a(21) =1183a(22) =1594a(23) =2152a(24) =2924a(25) =3961a(26) =5342a(27) =7230a(28) =9810a(29) =13265