Senegalise citizen with viza for china (study viza) do need visa for macao?( traveling purpose)

You need a visa to visit Macau, but the visa can be got at the entry port on your arrival.

and how many days am i allowed to stay there minimum?

You will be granted “Authorization to Stay” for a maximum of 30 days once the application is approved.

so..is there any kind of visa that allows you to stay there just for 15 days?

You were given a duration of 30 days, which means you can stay in Macau for at most 30 days instead of saying that you must stay in Macau for 30 days, neither more nor less. In other words, you can exit Macau on any day within 30 days with the 30-day duration visa.