1you are allowed to have as many jobs as you want in Germany. I am sorry for the misinformation other users spread here.
Working in multiple jobs is, unfortunately, bound to some rules. It is Germany, after all.
For once, any mini job can not interfere with your full time job. So if you turn up tired or exhausted at your full time job, your employer can veto the mini job. Moreover there is a legal limit on how many hours you are allowed to work. Your working hours can not exceed an half year average of 8 hours a day on six days (Monday through Saturday) of the week. So 48- Hours a week. This can be expanded to 10hours a day, so 60 hours a week, temporarily. But, and this is important, it still has to hit the average of 48 hours a week over a half year. So, if you work more than 48 hours in one week you have to work less in the next weeks. Another exceptions are jobs on sundays. Because you, better your employer, needs special permissions for those, same goes for jobs at night. Please tell us if you have a mini job at night or on a Sunday.
Taxes are another huge point to consider. The first mini job, the one you took up first, is free from social security tax, but that is optional. Every further mini job will just add income to your main job. So you have to decide whether it is worth it.