There is no limit to the number of tags that can be applied per asset, but between 15 and 30 tags is typically sufficient to ensure that your assets appear in relevant search results without drifting too far into the fringes. To add tags to single assets or to bulk batches of assets, follow the steps below.
Add Tags Individually
Click on an asset.
Type your new or existing tag into the section called Tags, located in the upper left.
Press Enter on your keyboard when you have finished typing an individual tag.
Tip: Vault will autofill related tags as your type. If you see the tag you want, click it.
Bulk Add Tags
Select your assets.
Click More Actions in the toolbar at the top.
Select Bulk Update Meta from the dropdown menu.
Enter your tags under the Tags section.
Remember to check the "Overwrite Tags?" option if you want your new tags to replace any existing embedded tags.
You should notice a field titled Shared Tags in the Bulk Edit modal window. Shared tags offer up a tally of the existing tags on the assets, with numerical counters for how many of the selected assets contain which tags. This makes it easy to spot at a glance which tags you need to add to all and which ones you've already taken care of.
In the example here, you can see that 2 of the 5 dog photos I selected have already been given the tag "hotdogs." However, because this tag should be applied to all the photos of dachshunds I've selected, I'll be sure to add it to the Tags box.
Also note that there is an option to specify tags you want removed from the assets, which is helpful if you need to undo a previous bulk action gone wrong.