How many shifts per week or month is normal for an attending?
It varies. First off, you should know that different ER's have different shift lengths, and this determines how many shifts per week (or month). Generally, shifts are either 8, 9, 10, or 12 hours long. You can even find some rural hospitals that have 24 hour shifts, but this is rare. In general, the busier the hospital, the shorter the shift (i.e. on the 8 hour side).
Generally, ER doctor's work in the range of 120-160 hours/month.
So, if you're working 8 hour shifts, then it's 15 shifts/month on the lower end, and 20 shifts/mo on the higher end (which is suicide in my opinion).
On the other hand, if you are working 12 hour shifts, then it's 10 shifts/month on the lower end, and 14 shifts/month on the higher end.
You can even work five or six 24-hour shifts per month, and have the rest of the month off.
In reality, you can work as much or as little as you want: most of us get paid per hour of work. So, figure out how much you want your yearly salary to be, then divide by your hourly rate. That's how many hours you need to do in the year to reach your goal salary. Just divide that by 12, and that's how many hours you need to work per month. It's quite simple.
Note: To be on staff you usually have a minimum number of shifts required per month. You can, however, avoid this by being per-diem, IC, locums, etc etc, all of which would allow you to work even fewer shifts if you wanted to. However, generally it's tough to keep a gig unless you commit to a certain number of shifts per month.
Also, keep in mind that on their "off" days, many ER doctors pick up extra shifts at other hospitals, work in urgent cares, or do other stuff.
There is tons of flexibility in EM, which is quite nice.