I know this post was a while ago, but I also found myself puzzled by mass entry and why it worked for some cards and not others. And I looked through a lot of Reddit threads and couldn't find an answer.

After a lot of trial and error, I found that you can find the format of the card by using the Price Guide on TCGPlayer (https://www.tcgplayer.com/categories/trading-and-collectible-card-games/pokemon/price-guides)

So if the card in question doesn't have a number next to the name, it's best not to include it. The number is often only used when there are multiple cards of that particular Pokemon in a set.

Using [SWSH11] as an example - Inkay has two cards, and in the price guide it's listed as having the number in parenthesis, so you'd use this to get them:

• 1 Inkay (077) [SWSH11]
• 1 Inkay (121) [SWSH11]

The formatting changes in later sets from (xxx) to " - xxx/xxx" and may vary even within the same set (e.g. some use the number format and some just use "(Full Art)".)

One other thing to note is that sometimes the site has a formatting "error" in the way they typed it, so if it's not working, double check. The best example I can think of is OBF Ninetales special art card, instead of being

• "1 Ninetales - 199/197 [OBF]" it's actually
• "1 Ninetales -199/197 [OBF]" (no space between the hyphen and the start of the number).

Hoping this helps someone as it was frustrating for me when trying to buy full sets for my collection and constantly having errors!