I was just in Germany visiting a friend of mine in early January and I have the chance to go back again in early March. Is there an amount of time I have to wait between visits even though I only stayed for a week?
You have to follow the 90/180 rule. At any day you're in any Germany and not just when you enter you must have been no more than 90 days out of the previous 180 in any Schengen state. (Partial days count.)
You cannot work in Germany, you have to be able to afford the trip and have either cash on hand or access to funds.
No, there is not. As long as you comply with the Schengen 90/180 rule, you can return the day after you left.
If your week-long stay began on a Saturday and ended the following Sunday, you used 9 of your 90 days. That means that if you return within 90 days of your departure, you can only stay for 81 days on your second visit. If you stay away for longer, your second visit can last for up to 90 days.
So, as long as you're not planning to stay for longer than 11.5 weeks, you should be fine.
As other answers have stated, the only restrictions are the 90/180 day rule and visa-free entry conditions such as not working. If you are planning repeated trips the Schengen short-stay visa calculator https://ec.europa.eu/assets/home/visa-calculator/calculator.htm?lang=en will help you figure out valid length of stay.
To answer the title: How many times can I travel between the US and Europe in a year? where Europe means the Schengen zone. An awful lot. You could fly DY 7700 from JFK to AMS arriving at 1pm and come back with KL 643 the same day. Go home, rest 24 hours. You spent one day inside Schengen (well, in reality, it's unlikely you had much time outside the airport, but hey) and one day outside. Repeat, rinse until the border guard gets bored of you and stops the pendulum. So the answer is 182 (or 183 on leap years).