-mila is ONLY used as a suffix, as in:
diecimila = ten thousand
tremila = three thousand
centoventimila = 120 thousand
The meaning is mille (one thousand) in numbers greater than 1000.
There is not a word like /mila/.
Is /mila/ singular or plural?
It is always used in a plural context, as /-cento/ is used in words like
trecento (300)
quattrocentocinquantadue (452)
But /cento/ is also a word by itself!
Example:
a) quanto costa questo quadro?
b) Cento euro. (One hundred euros)