Character names can be an easy place for readers to get lost during a plot summary. Some books will have very normal names, names that your reader may have trouble keeping track of. (Was Mary the mom, the sister, or the evil cheerleader?) Other books may have very unusual names, which can add additional confusion. (Is Golodrim a person or a place?) Further, most character names convey important meaning to you, who has already read the book to the end, but the names mean nearly nothing to readers of your plot summaries. Perhaps your main character has a on-again-off-again love interest or a best friend they've known since they were six. Using their names won't convey why a fight with Rebecca is a big deal or why Todd being kidnapped is tragic.
All of these problems can be solved by avoiding character names as much as possible. Most plotlines can be described by using only one or two names.
Naming the protagonist is usually preferable, since it creates a closeness and point-of-view to the summary. It's also usually necessary, because the protagonist will come up so often and need to be referred to as concisely as possible. You don't need to introduce their full first and last name though. Aside from being unnecessarily formal, using a character's full name will add extra words, which you can never afford to waste. So only the protagonist's first name or nickname should be included in your plot summaries.
Sometimes there is a need to give an additional character a name as well. This usually happens when another character is an important force in the plotline and the stakes. For example, love interests often need to be named in romance novels. Your story may have an antagonist that shows up to cause problems throughout the summary or the main character may be driven by the need to rescue or protect a loved one. When a character shows up three or more times in a summary, they may be easier to refer to by their name.
But in general, it's best to avoid using character names in a plot summary. So how can you paint a picture of your plotline without naming your characters?
Use a short description instead of the side character's name. How is this character related to the main character? Instead of a name, refer to the character as the protagonist's mother or best friend. If they aren't personally tied to the protagonist, you can refer to them by their role in the plot: for example, the king of the invading army. You may find that you are already using these type of descriptors to introduce your characters and can just remove the character's name. If you aren't, these descriptors likely will help readers follow the summary more easily than the names would.
Overall, that should leave you with one to two named characters. This should be a manageable amount for an agent or editor to read and absorb and still follow the plot points of your summary.
Keeping your plot summary easy to understand is really important in a world where agents read fifty queries in the same day and readers are sorting through thousands of book. Anything that confuses them or slows them down may make them lose interest in your book entirely.
To help you find a good balance, friends who aren't familiar with your book can be a huge help. These fresh eyes can easily identify any names that will be confusing to an agent, editor, or buyer. The more you can replace the confusing terms with common words, the better you will be able to keep the word count down and keep these plot summaries easy-to-read.