てもいい + a counter?

Question

I still can't get my head around how I am supposed to place the counters.

According to my textbook (which of course may have oversimplified some things), there are two possibilities: counter + の + noun OR particle + counter + verb.

But what if there's no verb (only the optional copula です)? I'm specifically interested in the construction te-verb + もいい(です).

Of course most of the time I can just attach it to the noun

何枚の写真を撮ってもいいですか - How many photos am I allowed to take?
五枚だけの写真を撮ってもいいです - You're only allowed to take five photos.

but what if I can't, since there are no nouns?

歌っても一回いいです (?) - I'm allowed to sing [it] once.

Is the above sentence correct? Should I treat いい as a verb (similarly to 好き)? If you know any resource explaining this in depth, linking it would be appreciated.

BTW Feel free to correct my sentences. I've been only learning Japanese for 2 months, so I expect there to be mistakes.

Answer 1

歌っても一回いいです (?) - I'm allowed to sing [it] once.

This is not grammatical. て(も)いい needs to follow a verb, so you'd say:

一回歌ってもいいです。

using 一回 adverbially. Alternatively:

一回だけ歌ってもいいです。 You're allowed to sing just once.
一回だけなら歌ってもいいです。 You're allowed to sing if it's only once.

Without using the verb 歌う, you'd say:

一回ならいいです。It's okay if it's once.
一回だけならいいです。 It's okay if it's just once.

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何枚の写真を撮ってもいいですか - How many photos am I allowed to take?

This looks okay. It might be a bit more natural to say:

何枚写真を撮っていいですか。
写真を/写真は何枚撮っていいですか。

using 何枚 adverbially.

５枚だけの写真を撮ってもいいです - You're only allowed to take five photos.

This doesn't sound natural. It'd be natural if you said:

５枚だけ写真を撮ってもいいです。
写真を/写真は５枚だけ撮ってもいいです。

with no の, using ５枚だけ adverbially.

As a response to the question 何枚写真を撮ってもいいですか, you'd usually leave out 写真を and say:

５枚なら(撮っても)いいです。You can (take) if it's five.
５枚だけなら(撮っても)いいです。 You can (take) if it's only five.

or, using まで "up to":

５枚までなら(撮っても)いいです。 You can (take) if it's up to five.
５枚まで(撮っても)いいです。You can (take) up to five.