Meanwhile I've checked the tax office's list on 1% offerings - taypayers can donate 1% of their personal income tax to a charity, and another 1% to a church, or alternatively, to a secular cause offered by the state (for 2009 that was eradication of ambrosia, a highly allergen plant, and a programme to support especially talented kids).
This list includes of course taxpayers only (i.e. people who have a legal income and pay at least minimal taxes), so it does not include for example retired people who have no other income than their retirement benefits. But it still gives a clue about how many adherents churches have.
At present, one needs 100 members to found a church, and though the "statues" (main teachings etc.) have to be handed at registration, the state has no right to check them in any way.
The tax office lists 185 churches for 2009, the biggest one being the catholics, with 534 thousand people offering their 1% (45% of onepercents dedicated), and on the second place, lagging far behind, the reformed church with 186 thousand (20%). The third 'big traditional' church, the Lutherans has 49 thousand taxpaying members (4%). Another 3 make it over the 10000 adherents limit that Szászfalvi has suggested should be introduced: the baptists (1.3%), the Krishnas (1.1%) and a pentecostal church called Hit Gyülekezete (the Faith Church, 1.6%).
Of course as probably not only the taxpayers would count, some more churches will meet the criterion, such as the biggest Jewish and buddhist groups, as well as Jehovah's Witnesses. Overall, there are 25 churches that got more than 1000 donations, and only 15 of them have over 2000 income tax paying members who are devoted enough to fill out two lines on a form. Of these 25, 13 are Christian, 5 Buddhist, and 2 Jewish. Among the remaining, there are the Scientologists, the Jains (who are basically supporting an animal shelter called Noah's Ark), the Krishnas, an esoteric church and another with the aim to reestablish giving birth as spiritual act (supposedly related to the fact that anyone who would like to give birth at home is considered crazy by the medical establishment). Of course some more churches can count on retaining their status as 'established', nemaly those who have been present 'traditionally'. I can only guess about them, but I suppose Jewish and Christian orthodox groups (e.g. the Greek and Serb churches) will be among them, and the muslims will prbably argue that they are traditionally present in Hungary (after all, we had 150 years of Turkish rule, hadn't we...).
The list also contains the total amount of 1%-s donated to the curches, so one can count the avarage personal income (or rather, about the income tax) of the members of one particlar church. Among the over 1000 sized churches, the biggest taxpayers are (reinforcing stereotypes) the Jews (the avarage 1% of the income tax is 5900 forint per taxpayer, while Jews' 1% was 12509 forint for the biggest Jewish group and 10321 forint for the 'Status quo ante' group - the two groups separated in the 19th century over an argument on the relationship with the state).
The by faaar richest church members are Christians, however: the 84 members of the Korean Christians' 1% amounted to 58385 forint per person, which is ten times the avarage income (no typo, the amount was similar last year).
Before I forget: 25% of taxpayers donating their 1% gave it to the secular causes. Their avarage income avarages the general avarage.
But: most taxpayers just don't care. There are about 3.6 million taxpayers, of whom 24.8% are close enough to their church to donate it ca. 2 minutes to fill out the form (for the big denominations: 14.8% catholics, 5% protestant, 1.4% Lutherans, with the rest under 1%), and another 8.3% are dedicated enough to a quite impersonal secular cause (getting no media coverage) to offer their 1%. (Data corrected on 23/10/2010 23:32, after I've found the exact number of persons paying actually more than 0 forint taxes.)
The list: http://www.apeh.hu/szja1_1/kimutat/2010_rend_egyh_kiem.html