FWIW, this should be in the Discussions and Requests sub-forum.
Anyway - this is documented in the vanilla spawning.xml file:
<!--
biome
maxcount: byte - the sum of all spawned entities in the group may not exceed this number in a 5 chunk by 5 chunk (80m x 80m) "biome chunk"
respawndelay: game days - .000278 real seconds, .0167 mins
daytime: Any,Day,Night - determines the time of day the entity spawns
tags: restricts the spawn to areas that have any tile/POI with those tags
notags: restricts the spawn to areas that don't have any tiles/POIs with those tags
(...the rest refers to info that is no longer used...)
-->
A respawn delay value of 1 means "delay one game day." The lowest vanilla respawn delay value is 0.1, which means 10 per game day will spawn.
This is up to the max count, but the max count is per 80m x 80m square area, which is not a very big area.
However - NPC core, and most NPC Packs, also spawn "nothing" 97% of the time. This is specifically to prevent over-spawning. In the entity groups representing the biome spawns, you will find this entry:
none, 97
It is probably easier to lower this number if you want more NPCs to spawn. They will still spawn only one per 80x80m area, but that will mean more areas will spawn them.
Here are some example values:
- 64: ~4.5% chance to spawn NPCs, or 1.5x the existing spawn rate
- 47: 6% chance to spawn NPCs, or 2x (double) the existing spawn rate
- 30: ~9% chance to spawn NPCs, or roughly 3x the existing spawn rate
- 9: 25% chance to spawn NPCs - this will spawn a lot of NPCs
You can experiment with either to spawn more NPCs.
Also, if you are using any of my NPC packs, there is commented-out XPath in entitygroups.xml that you can un-comment to accomplish this.
HTH.