确实两种形式似乎都可以~ 但是tattoos肯定是可以作为复数形式的

然后我记得我看过的美剧《friends》里第二季16集里有提到tattoo

具体人物对话剧本如下：

[Scene: Tattoo parlor. Phoebe and Rachel are deciding on tattoos.]

场景：刺青商店客厅内 Phoebe and Rachel 正在对选择那种图案的刺青作决定

PHOEBE: OK Rach, which, which lily? This lily or that lily?

RACHEL: Well I. . .

PHOEBE: I like this lily. It's more open, ya know, and that's like my mom. She had a more open, giving spirit. Ooh, Foghorn Leghorn, ooh.

TATTOO ARTIST（负责为顾客刺青的老头）: Alright, blonde girl, you're in room two, not so blonde girl, you're with me.

。。。。。（省略一部分）

PHOEBE: Just, you know, long time ago. Well, when men used to tell women what to do - a lot. And then there was suffrage, which is a good thing but is sounds horrible. Do you want to get this tattoos?

你想要这个刺青吗（最后一句）

先举例这几句 后面还有但同样可以总结出下面一点：

我列出的上面一共出现tattoo 四次

以tattoos 形式出现的：deciding on tattoos，to get this tattoos?

可见都是以单纯的名词形式出现（后面不加任何东西）

以tattoo 形式出现：Tattoo parlor， TATTOO ARTIST

这里都是与另一个名词一起组成一个新的集合名词

希望对你有帮助~