My work had a shower for me...there were 8 ladies there so I knew who to write thank you notes to.
Where I have been employed in the last few yeas and contributed to showers the MTB or BTB has just written one thank you card for everyone and brought in either donuts, cookies, bagels, etc. The card usually just says "From the ?? Group" so that those that didn't contribute (and there are always some) aren't made to feel bad.