actually i was looking at floater and i think floater drawing routine 1 must be the absolute simplest one for an alien with complete walking cycle turning and ability to hold weapons.
#1 has a 5 full body frame walking cycle for each of the 8 directions (n nw w sw s se e ne) . and has separate hands to hold objects . seems it has 6 x 8 = 48 frames for hands. i assume 8 are for not holding anything, 8 are for holding something and 8 are for attacking . twice that is 2 x 8 x 8 x 8 = 2 x 24 = 48.
Bit more complex.
8x2 for not holding anything, 8 for left arm wielding (any weapon), 8x2 for right arm wielding (1-handed wielding & 2-handed wielding), 8 for right arm firing 2-handed weapon.
Same goes for snakeman, as well as Muton (Muton is using the same routine as human).
Reference for human/muton/sectoid:
xcom_0232 - xcom_0239 right arm (one-handed weapon)
xcom_0240 - xcom_0247 left arm (any weapon)
xcom_0248 - xcom_0255 right arm (two-handed weapon)
xcom_0256 - xcom_0263 right arm (firing weapon)