I've now seen a physical copy of Barrett et al.'s World Christian Encyclopedia in the 2001 edition. Let's get two things straight right off. First, the word "sect" is not used, at least not in the tables whose numbers are being bandied about. These tables refer to counts of "denominations". Second, the 33,000 number is total Christian denominations, not just Protestants.
All this data comes from Volume I. It is essentially in two parts: a worldwide survey, and a country-by-country analysis. For each country there is a summary table, a nartive analysis, and a detailed listing of raw data by denomination. (The USA data cuts off for denominations with memberships under 1,000 and gives "other X bodies" estimates/counts for the smaller groups.)
Christianity as a whole is broken out into six "megablocs". Three of these are obvious: Orthodox, Anglican, and Roman Catholic. The remaining three are Protestant, Independent, and marginal. The last is just what it sounds like: non-trinitarians and other dubious groups (e.g. Mormons, JWs, Unitarians, but for some reason not the Quakers-- they are counted as Protestant). Protestant, at least in the USA, represents the main large bodies (e.g. ELCA, PCUSA, EMC...).
Independent, however, is full of surprises. It does not mean just unorganized baptist polity groups! In the USA data, all Old Catholics are in thie group, along with splinter Anglicans, Old Calendarists, Old Believers, as well as the independent baptist groups. I think most of the anabaptists are listed here too. I went through the raw data section and by my count there are 20 Orthodox bodies listed as Othodox, another 20 counted as "Other Orthodox", and another 7-9 as independents. They add the first two together in the summary to give a total of 40 Orthodox denominations in the USA. However, only ECUSA is counted as an Anglican body, but the REC and what continuing churches are big enough to make it into the list are all "Independent", and the 20 "Other Anglicans" are also totalled into the "Independent" summary total.
The world totals have another surprise. They list 237 countries total, but if you look under the RC column they count 238 RC denominations-- basically one per country. They apparently count each country with an Anglican presence in the same way, giving 168 Anglican denominations. I'm pretty sure the total number of communion members is somewhat less than that. All the big numbers are being hidden in the Independent megabloc, giving 2/3s of the world total. Africa supplies one third of the total denominations, and 83% of that is Independent. If you look at the USA numbers and compare 1975 to 2000, you find that by their reckoning all the increase in denominations is under Independent. Marginal groups increased by 55%, but they are not all that numerous; Protestant groups actually decreased by nearly 9% (but again, the numbers are relatively small-- essentially 20% of the total in 2000). There's also the problem of lumping together groups which are deliberately separate (e.g. REC or LCMS) with groups that are separate simply because they feel no need for any kind of unification (the various baptists). Counting the latter is problematic, particularly in Africa where a lot of the "division" is nothing more than the results of expensive or poor communication and travel. I would guess that in the USA, for example, a lot of the independent evangelicals do in fact form a single group which, however, is not represented in a political structure.
The raw data appear to be good, but these categorizations are highly misleading.