I sit corrected!
For the third time: The FIRST lyric of the song is a number. Not the chorus or the countdown, but the FIRST WORD IN THE LYRIC.
While “one” seems to the most popular (no surprise there) nobody has mentioned Brewer & Shipley’s classic drug song “One toke over the line sweet Jesus.”
…except it was mentioned in post #10. 
Honourable mention for ‘19’, by Paul Hardcastle?
“In 1965, Vietnam seemed like just another foreign war”
Another iffy one:
*Are You Gonna Be My Girl * by Jet …
“Well it’s a one, two, three take my hand an come with me …”
Here’s one that even helpfully points out that the first lyric is a number:
“1989 the number another summer (get down)” - Fight The Power by Public Enemy.
??? How can this be “iffy”???
The number one is the fourth word in the song, so the song doesn’t begin with a number.
Because, “Well its’ a-” is superfluous, just mumble mouth added to fix the meter.
But if you don’t want it to be, it doesn’t have to be. Don’t want you to have a coronary or anything.
I’m just responding to the OP’s intent. The OP stated that it was songs that start with a number, clear and simple, but I’ve seen so many posts of songs that don’t start with a number. I don’t understand how that can be missed by so many…
“Three little words, eight little letters…”
An obscure song–it’s been turning up a lot on one of my wife’s Pandora channels, but I’ve never encountered it anywhere else: “Snow Mazes of Norway,” by indie singer/songwriter Jenny Dalton.
“Six points on a snowflake, eight arrows on a compass…”
Okay, okay. Just sit back and relax. Here have a Zweiback. Jeez, you try to have a little fun.
Is there an Internet Law about this a la Gaudere’s? If not, there should be. Seems like any thread I go into of reasonable length has someone at the end saying “I can’t believe nobody’s mentioned YADDAYADDAYADDA,” and YADDAYADDAYADDA has, in fact, been mentioned within the first ten (or pick a relatively low number) posts.
1,000 umbrellas
Upturned couldn’t catch all the rain
That drained out of my head…
- XTC
I don’t think that works: the first word in the song is “I’m”.
One, Two, Buckle My Shoe
“Seven deadly sins
Seven ways to win
Seven holy paths to hell and your trip begins”
– “Moonchild”, Iron Maiden
“Seven
seven
seven deadly sins…”
-“Seven Deadly Sins,” The Traveling Wilburys
One Of These Days–Ten Years After
Four years down,
and twenty-one to blow;
ten thousand more breakfasts to go…
(Lee Hazelwood, Pray them bars away).