You don't ever have to as feedback is voluntary (not compulsory). The seller will only receive 1 feedback increase whether you comment on 1 or 50 items. However, it does increase their items bought/sold count. If you are happy with all of the items, write one generic comment such, "All items are of excellent quality, thank you for the fantastic service". If you really appreciate the seller, copy-paste the comment (doesn't take that long) for each item you bought. This will increase the items count for this seller.
eBay allows for separate comments for individual items. 49 might have been excellent, whilst one might have been disappointing or incorrectly described, and you might wish to follow this up with the seller prior to leaving feedback for that particular item.
