How many people speak Esperanto, Ido, Occidental, Interlingua, Lingua Franca Nova, and so on?

Nobody knows. There are discussions about this on Auxlang all the time, and the general consensus is that it goes like this:

• Esperanto: 100,000 - 2 million
• Ido: 1,000 - 2,500
• Interlingua: About the same as Ido
• Occidental, Lingua Franca Nova, Lojban, Novial, etc.: About 20-100 each.

There's another way to judge the relative amount of interest though, and that's by the number of users on the English Wikipedia. Users are able to put infoboxes on their user pages to show what they are interested in, what software they use, what languages they know and how good they are at them, or anything else. My user page for example looks like this:

en	This user is a native speaker of English.

ja-4	この利用者は母語に近い水準で日本語を話すことができます。

ko-3	이 사용자는 한국어를 유창하게 할 수 있습니다.

io-3	Ica uzanto povas kontributar kun alta nivelo di Ido.

zh-2	該用戶能以一般的中文進行交流。 该用户能以一般的中文进行交流。

fr-2	Cet utilisateur peut contribuer avec un niveau intermédiaire en français.

no-2	Denne brukeren har god kjennskap til norsk.

ia-2	Iste usator pote contribuer con un nivello intermedie de interlingua.

eo-2	Ĉi tiu uzanto povas komuniki per meza nivelo de Esperanto.

tr-2	Bu kullanıcı orta seviyede Türkçe anlıyor.

es-1	Este usuario puede contribuir con un nivel básico de español.

af-1	Hierdie gebruiker het 'n basiese begrip van Afrikaans.

et-1	See kasutaja suudab eesti keeles kaastööd teha algaja tasemel.

de-1	Dieser Benutzer hat grundlegende Deutschkenntnisse.

bg-1	Този потребител има начални познания по български език.

That means that I'm registered as being a user of Ido, Esperanto and Interlingua. Now let's take a look at how many other people have done the same:

(total of 551 users)

• Esperanto: 338 users (61%)
  
• Interlingua: 71 users (13%)
  
• Lojban: 61 users (11%)
  
• Ido: 35 users (6%)
  
• Volapük: 20 users (4%)
  
• Occidental / Interlingue: 14 users (2.5%)
  
• Lingua Franca Nova: 8 users (1.5%)
  
• Novial: 4 users (0.7%)
  

It's not quite accurate because if a user puts an infobox on a sub-page it will also count as another entry, and you can see this in a few cases in Volapük for example.