What is it?

Excuse me. What is its name? Thanks.

Good Question! Are you sure it is a Mammillaria in the first place.

A clerk told me that it is a Mammillaria, but he don't know its name.

I have two suggestions:

First, could it be a form of M. decipiens, with no central spines?

Second, could it be Escobaria vivipara, maybe v. bisbeeana?

Has it flowered yet?

The second is my preferred answer, but I really am not sure.

I am not sure, too.
There are two features of it.
Maybe that will help you.
1.There are some fine hairs on its sticks.
2.Its buds are white.

With white buds, it isn't an Escobaria, so I was wrong with that suggestion.
BUT I still think that it might be a form of M. decipiens although it doesn't match precisely the description for ssp. albescens. The description of this specifies 3 to 5 radial spines. This plant has rather more!

I to was thinking along the lines of Escobaria, something like missouriensis ( Neobesseya ).
I would not rule it out yet, even with the flower buds which are looking white at the momment. Its still early days yet.

I agree with Chris. My money is on M. decipiens, with no central spines.

Take a look at maurillio's excellent pics.... particularly the second and third photos (imagine these plants without centrals... looks familiar?)https://mammillaria.forumotion.net/starting-with-d-f25/mammillaria-decipiens-t1080.htm

I don't believe that it is M.decipiens. It hasn't pubescens spines.
Please compare with M.picta there are forms without centrals.

Corect: M. picta

mammillaria picta....

Thank you.

Flower

Yes, I would also vote for M. picta, and to be more precise M. picta ssp. viereckii. Attached is a photo of a plant that I found in 2009 near Balneario de Nogales, somewhat east of Jaumave in the state of Tamaulipas.

Wolter.