a.
Jüri can draw two lines and draw any number of circles such that they touch both of the lines.

Fig. 8

b.
Assume that Jüri has solved the problem for some $n$, where $n > 1$.

Let $O$ be the intersection point of all the lines. Look at any circle $c$. From the premises of the problem we see that the circle $c$ touches two of the lines drawn by Jüri, which we call $k$ and $l$. But any one circle can only touch up to two lines drawn from one point. So, the circle $c$ does not have any more lines touching it. If $c'$ is any other circle drawn by Jüri, then the common tangents of $c$ and $c'$ can only be $k$ and $l$. Therefore, $k$ and $l$ are the common tangents of all the circles. Two lines divide the plane into four sectors, inside each can be only one circle with the previously set radius.

Fig. 9

So, for $n > 4$ the problem has no solution but for $n \le 4$, it obviously has.