Hi Johnnie and welcome. This will depend on the manufacturer and shot size. A general guideline for a 12ga 1-1/4 oz load is:
#1 75
#3 117
#5 157
#6 225
#7.5 253
#8 281
#9 337
This should not vary much past 3% amongst manufacturers as some shot holders may be thinner or thicker or a completely different design. Cheers, stay safe and enjoy!

I'm figuring you are asking for self-defense purposes, because who really is worried about how many pellets are in birdshot, you are planning on most of those missing anyway. Right now I'm looking at a round of federal 2 & 3/4" tactical, 00 buck right now. There are 9 pellets in the shell.

A 2-3/4" 1-1/4 oz lead shotgun shell contains approximately 150-170 pellets which depends greatly on the pellet size. For example: 169 pellets for #4 shot or 210-224 pellets for #5 shot. Exact numbers vary based on different shot sizes.
#4 shot has a 1-1/4 oz load and approx 169 pellets. #5 shot has a 1-1/4 oz load amd approx 210 to 224 pellets. 🔥❗️

A 2-3/4" 1-1/4 oz lead shotgun shell contains approximately 150-170 pellets, depending on the pellet size, such as 169 pellets for #4 shot or 210-224 pellets for #5 shot. The exact number varies because different shot sizes have a different number of pellets per ounce, with larger pellets resulting in fewer pellets in the total payload.
For #4 shot: A 1-1/4 oz load will have roughly 169 pellets.
For #5 shot: A 1-1/4 oz load can contain approximately 210 to 224 pellets.