"I am thinking of a number from one to ten."
Translation:Myslím si číslo od jedné do deseti.
It is the genitive case of the feminine gender.
We normaly refer to number one as feminine "jedna". But sometimes you will also encounter masculine "jeden". When counting up one, two, three,... both genders are possible.
I do not understand the presence of "si" here. I would have answered Myslím číslo... as I do not see "reflexive" mode of verb think.
What do you mean "I do not see"? It certainly exists and belongs here. And it is common, in fact: "Myslím si, že je to dobré." "I think it is good."
Non-reflexive myslet is usually used with a preposition. "I am thinking about you." "Myslím na tebe."