With continuous sparging, there is an optimal volume of wort that can be collected from a given weight of malted grain. If a brewer collects less than this amount, he is leaving sugars behind in the grain bed. If he exceeds this amount, he will get more sugars, but at the expense of extracting excess tannins from the grain husks, which will make his beer astringent. On my system, this happens around 0.65 gallons of wort per pound of grain. The exact volume varies (although not by much) and this is something a serious continuously-sparging homebrewer should determine for himself on his own system. But what about batch spargers?
There are at least a few different ways that batch spargers approach wort collection. The simplest possible way to batch sparge would be to mash at a reasonable mash thickness, recirculate and then drain the first wort. Then, the first dose of sparge water would be added such that the liquid level in the mash/lauter tun equals the level in the first mash. Then the wort would be recirculated and collected again. This would yield first and second worts of equal volume. This could be repeated once more to yield a third volume of wort.
At a mash thickness of 1.25 quarts of water per pound of grain (2.6 L/kg), and assuming the grains absorbs 0.12 gallons of wort per pound, the amount of wort collected is 0.39 gallons per pound for 1 dose of sparge water or 0.58 gallons per pound for 2 doses of sparge water. (This also assumes no dead spaces in the mash/lauter tun.) If you decrease the mash thickness to 1.5 qts./lb. (3.1 L/kg), the amounts of wort change to 0.54 gallons per pound for 1 dose of sparge water (and 0.80 gallons/lb. for 2 doses; this is likely to be too dilute in many cases). Whatever the values, the amount of wort collected would be tied to the amount of grain in the grist. As a consequence, your extract efficiency would be the same no matter the size of your grain bill.
At lower grain bill weights, the brewer may need to add water to either the kettle or to the mash and sparge water volumes to yield a reasonable pre-boil wort volume. And this is common practice. Frequently, after the mash is finished, an additional dose of water is added before draining the first wort. Sufficient sparge water is then added so that the volume of second wort equals the volume of first wort collected and the target pre-boil volume is obtained.
No one knows how much wort can be collected by batch sparging from a given weight of grain without extracting excess tannins. However, sticking to a maximum of around 0.65 gallons/pound (the continuous sparge ratio) should be safe. To do this, multiply the weight of the grain bed (in pounds) by 0.65 gallons/lb. to yield the volume of wort to collect. (Or, use the table I posted yesterday.) If this is lower than a reasonable pre-boil volume (one that can be boiled down to your batch volume in 60–90 minutes), add water to the kettle to yield a reasonable amount. Given the mechanics of batch sparging, it’s possible you could collect more wort and not extract excess tannins, but this should give a limit that’s known to be safe.
At higher grain bill weights, the wort can be boiled for extended amounts of time. Or, the brewer can limit the number of times sparge water is added or the volume of sparge water to yield a lesser pre-wort volume. Lowering the amount of sparge water, however, will also lower your extract efficiency.
So, if you collect your wort by batch sparging, with a reasonable mash thickness you can collect either two or three worts from your grain bed and not oversparge the grains. (The amount of sugars recovered with the second dose of sparge water is low and many batch spargers skip the collecting the third wort, even though it lowers their extract efficiency slightly.) By adding water to the mash and sparge water additions, you can hit a reasonable pre-boil wort volume when you have a smaller grain bed. For very small grain beds, adding water to the kettle may be better. If you are mashing a lot of grain, you can either boil for an extended amount of time or limit the number or volume of sparge water additions to lower the pre-boil volume and shorten your boil time.