I haven't really studied this, but have found that No. 4, 5 and 6 taps all take the same driver, It is possible that other, larger size taps also come in groups, like maybe 1/4 and 5/16. something to find out about before buying tap driver collets.
Thanks for the info. Following your lead I went internet hunting and found a tap chart that shows size 0 thru 6 are the same shank size. After that shank diameters are different for each size up to 1/2" which is the max size I plan on using. I need to compare tap shank sizes to ER20 collets to see what makes sense.