I had frozen takioyaki for today’s lunch.

Each of thempiece is small, so I ended up eating as many as forty40 takoyaki.

"Forty" is fine, but I think it's more natural to write it as a number.

We usually love to put bonito flakes, and okonomiyaki sauce on our takoyaki, and s. Some people also put mayonnaise on them.

I'll go get some takoyaki (outside/from a takoyaki shop) next time.

I had frozen takioyaki for today’s lunch.

We usually love bonito flakes, and okonomiyaki sauce on our takoyaki, and some people also put mayonnaise on them.

In my case, I don’t putadd mayonnaise.

If you use put, you will need to add "on" to specify where you put it.

Each of themne is small, so I ended up eating as many as forty takoyaki.

“Each one of them” can work here too.

We usually love bonito flakes, and okonomiyaki sauce on our takoyaki, and s. Some people also put mayonnaise on them.

The comma separated list here raises a slight warning in my head when I read it. I think it’s because the first two points of the list are things that you are doing, and the last point is something that someone else might do. So it’s probably better to break it into two sentences.

I'll go get somet takoyaki again next time.

I had frozen takioyaki for today’s lunchlunch today.

I initially confused this for "taiyaki", but then noticed the order of the k and i were different. Looking at the Japanese confirmed that this was meant to be "takoyaki"

"today's lunch" is fine grammatically, but it is a lot more common to say "for lunch today" in my opinion. For the title, "Today's Lunch" works perfectly, however.

Each of them is small, so I ended up eating as many as forty takoyaki balls.

Takoyaki is often used as a collective noun, but it sounds a little strange if you refer to a single one as "takoyaki". Therefore we'd usually talk about a "takoyaki ball" if talking about them individually.

(Japanese people/We) usually lohave bonito flakes, okonomiyaki sauce on our takoyaki, and some people also put mayonnaise on them.

"love" sounds a little strange here.
I'd also say that just saying "we" would be quite unclear unless it was already apparent from prior sentences that you were talking about Japanese people in general.

In my case, I don’t put mayonnaise on them.

"put" requires a preposition (in this case "on") - you cannot drop it here

These days, there are many kinds of takoyaki, like “ネギマヨ”, “明太子マヨ” etc.

No real issue here, just want to say that for people who don't read Japanese this would maybe be difficult to understand
ネギマヨ is spring onion and mayo, while 明太子マヨ is mentaiko and mayo

I'll go get some takoyaki next time I'm at the shop.

An English-speaker reading this would probably be wondering "next time you do what?". Unless it was clear before you should probably include it.

I had frozen tTakiyaki for today’s lunchlunch today.

Because Takiyaki is a specific dish, the name is capitalized. "For lunch today," flows better than "for today's lunch," but it is correct either way.

Each of them is small, so I ended up eating as many as forty tTakoyaki.

We usually love bonito flakes, okonomiyaki sauce on our tTakoyaki, and some people also put mayonnaise on them.

These days, there are many kinds of tTakoyaki, like “ネギマヨ”, “明太子マヨ” etc.

I'll go get some tTakoyaki next time.