In a custom loot table containing the following, a mob assigned this loot table will drop only one totem of undying. Drops of multiples for other items works correctly.

{
"pools":
[
{
"rolls": 1,
"entries":
[
{
"type": "item",
"name": "minecraft:totem_of_undying",
"weight": 1,
"functions":
[
{
"function": "set_count",
"count":

}
]
}
]
}
]
}