If you have done starters from a while labs vial, then you can estimate the cell count based on the size of the harvested yeast cake.
I have never used a starter on harvested ale yeast. These yeast are already healthy, likely a good cell count and ready to get into the action right away.
My process and experience is below.
I use harvested yeast often. My process is to boil then cool to 60F 1 liter water in a 2 Liter Erlenmeyer flask loosely covered with foil. Xfer yeast and trub from a previous batch adding about 500ml -1 liter. Foil only comes off during the xfer.
In a 1 Liter Erlenmeyer flask, I boil 500 ML water covered loosely with foil and cool to 60F.
I slowly decant off all the liquid in the 2 liter until the yeast start to escape. I let it settle for 15 - 30 minutes then slowly transfer the yeast into the 1 liter. Once trub starts to transfer, I stop.
From the 1 Liter flask, I decant off the water and pitch the rest into 5.5 gallons of wort. I do this for Imperial IPAs, Imperial Porters and Barleywines. I have excellent fermentation and complete attenuation.
My brew on Sunday, I used a siphon cane and transferred 750 ML of the yeast/trub cake from a porter I brewed a week ago into a flask, let it settle at round 60F (garage temp) for a couple hours, decanted off the beer and pitched about 100ML of the harvested yeast/trub into 1.071 gravity IPA wort. When I checked the next morning, fermentation was going well and had been rocking since yesterday afternoon.