Is zakat obligatory on a publicity and advertising agency from which its owner saves nothing, whose fixed capital is 20,000 Saudi riyals and whose circulating capital is 500 Saudi riyals, and which contains no goods? And what is the amount of this zakat if it is obligatory?

As long as you spend what you earn from the shop on your living expenses and nothing remains with you from it for a full year, and you have no other zakatable wealth, then there is no Zakat due on you. Zakat becomes obligatory if you keep any of it until a full year has passed over it, and it reaches the nisab either by itself or by combining it with other wealth. As for the tools and the shop itself, they are not subject to Zakat; rather, their output is zakatable if a full year passes over it and it reaches the nisab.