I cannot tell you what capacity pump you need. Too many variables. What size bowl, how much piping, are you using standard copper fittings vs e.g., Swagelok, how good is the seal on the adapter for the headstock?
If the piping system and seal of the wood were perfect, then capacity would not matter, a smaller unit would just take longer to extract all the air than a larger unit.
In practice, there are always leaks. The wood itself may be porous. The more leakage, the more you want a higher capacity pump than a lower capacity.
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