Sure, a first person narration is going to use “I" often and many of these uses will happen at the beginning of sentences. But you can overuse “I" to the point of monotony. Don't get hung up on counting sentences, though. Some paragraphs will simply demand more sentences that begin with “I" and some less. Instead, focus on the flow of the story. In Edwidge Danticat's short story “Night Women," a woman prepares for her nighttime “suitor" while her young son sleeps:
I whisper my mountain stories in his ear, stories of the ghost women and the stars in their hair. I tell him of the deadly snakes lying at one end of a rainbow and the hat full of gold lying at the other end. I tell him that if I cross a stream of glass-clear hibiscus, I can make myself a goddess. I blow on his long eyelashes to see if he's truly asleep. My fingers coil themselves into visions of birds on his nose. I want him to forget that we live in a place where nothing lasts. I know that sometimes he wonders why I take such painstaking care. Why I draw half-moons on my sweaty forehead and spread crimson powders on the rise of my cheeks. We put on his ruffled Sunday suit and I tell him that we are expecting a sweet angel and where angels tread the hosts must be as beautiful as floating hibiscus.
Six of the eight sentences start with “I" but the paragraph reads smoothly. In fact, this choice adds to the narrator's voice. Other paragraphs in this story don't have as many sentences that begin with “I" and some paragraphs have none.
If the narrative sounds off—and it's bound to if you're concerned about this—start revising. Look for moments where you can vary sentence structure. One unedited paragraph might look like this:
I had three dollars and fifty-one cents in my pocket. I dug out a cigarette while waiting for Jimmy. I thought he was taking too long. I noticed the sign above the diner. The “J" had burned out, so it just read “immy's." I smiled, thinking that it would tick off Jimmy even more if I said something about it.
By changing structure, the second sentence could easily read, “Waiting for Jimmy, I dug out a cigarette."
You may also find that some sentences simply don't need “I" and its accompanying verb. For example, “I thought he was taking to long," could be shortened to, “He was taking too long."
Also, look for sentences that could be combined or rearranged. For example, here are two sentences from the original paragraph:
I noticed the neon sign above the diner. The “J" had burned out, so it just read “immy's."
These could be combined into one sentence:
The “J" had burned out on the neon sign above the diner, so it just read “immy's."
Or they could be rearranged:
The neon sign above the diner read “immy's." The “J" had burnt out.
Here's a revised version of the whole paragraph:
I had three dollars and fifty-one cents in my pocket. Waiting for Jimmy, I dug out a cigarette. He was taking too long. The neon sign above the diner read “immy's." The “J" had burnt out. It would tick off Jimmy even more if I said something about it.
Make sure you're not just upending sentence structure and order for the sake of it. Some changes can actually sound worse. Always keep the overall flow of the paragraph in mind.