Schema:
CREATE TABLE PostLinks (
    Id number,
    CreationDate time,
    PostId number,
    RelatedPostId number,
    LinkTypeId number
)

CREATE TABLE CloseReasonTypes (
    Id number,
    Name text,
    Description text
)

CREATE TABLE SuggestedEdits (
    Id number,
    PostId number,
    CreationDate time,
    ApprovalDate time,
    RejectionDate time,
    OwnerUserId number,
    Comment text,
    Text text,
    Title text,
    Tags text,
    RevisionGUID other
)

CREATE TABLE PostsWithDeleted (
    Id number,
    PostTypeId number,
    AcceptedAnswerId number,
    ParentId number,
    CreationDate time,
    DeletionDate time,
    Score number,
    ViewCount number,
    Body text,
    OwnerUserId number,
    OwnerDisplayName text,
    LastEditorUserId number,
    LastEditorDisplayName text,
    LastEditDate time,
    LastActivityDate time,
    Title text,
    Tags text,
    AnswerCount number,
    CommentCount number,
    FavoriteCount number,
    ClosedDate time,
    CommunityOwnedDate time,
    ContentLicense text
)

CREATE TABLE ReviewTaskTypes (
    Id number,
    Name text,
    Description text
)

CREATE TABLE CloseAsOffTopicReasonTypes (
    Id number,
    IsUniversal boolean,
    InputTitle text,
    MarkdownInputGuidance text,
    MarkdownPostOwnerGuidance text,
    MarkdownPrivilegedUserGuidance text,
    MarkdownConcensusDescription text,
    CreationDate time,
    CreationModeratorId number,
    ApprovalDate time,
    ApprovalModeratorId number,
    DeactivationDate time,
    DeactivationModeratorId number
)

CREATE TABLE VoteTypes (
    Id number,
    Name text
)

CREATE TABLE Posts (
    Id number,
    PostTypeId number,
    AcceptedAnswerId number,
    ParentId number,
    CreationDate time,
    DeletionDate time,
    Score number,
    ViewCount number,
    Body text,
    OwnerUserId number,
    OwnerDisplayName text,
    LastEditorUserId number,
    LastEditorDisplayName text,
    LastEditDate time,
    LastActivityDate time,
    Title text,
    Tags text,
    AnswerCount number,
    CommentCount number,
    FavoriteCount number,
    ClosedDate time,
    CommunityOwnedDate time,
    ContentLicense text
)

CREATE TABLE PostNotices (
    Id number,
    PostId number,
    PostNoticeTypeId number,
    CreationDate time,
    DeletionDate time,
    ExpiryDate time,
    Body text,
    OwnerUserId number,
    DeletionUserId number
)

CREATE TABLE PostNoticeTypes (
    Id number,
    ClassId number,
    Name text,
    Body text,
    IsHidden boolean,
    Predefined boolean,
    PostNoticeDurationId number
)

CREATE TABLE Votes (
    Id number,
    PostId number,
    VoteTypeId number,
    UserId number,
    CreationDate time,
    BountyAmount number
)

CREATE TABLE ReviewTasks (
    Id number,
    ReviewTaskTypeId number,
    CreationDate time,
    DeletionDate time,
    ReviewTaskStateId number,
    PostId number,
    SuggestedEditId number,
    CompletedByReviewTaskId number
)

CREATE TABLE Comments (
    Id number,
    PostId number,
    Score number,
    Text text,
    CreationDate time,
    UserDisplayName text,
    UserId number,
    ContentLicense text
)

CREATE TABLE PostTags (
    PostId number,
    TagId number
)

CREATE TABLE PostHistoryTypes (
    Id number,
    Name text
)

CREATE TABLE PostHistory (
    Id number,
    PostHistoryTypeId number,
    PostId number,
    RevisionGUID other,
    CreationDate time,
    UserId number,
    UserDisplayName text,
    Comment text,
    Text text,
    ContentLicense text
)

CREATE TABLE TagSynonyms (
    Id number,
    SourceTagName text,
    TargetTagName text,
    CreationDate time,
    OwnerUserId number,
    AutoRenameCount number,
    LastAutoRename time,
    Score number,
    ApprovedByUserId number,
    ApprovalDate time
)

CREATE TABLE PostTypes (
    Id number,
    Name text
)

CREATE TABLE ReviewTaskStates (
    Id number,
    Name text,
    Description text
)

CREATE TABLE Users (
    Id number,
    Reputation number,
    CreationDate time,
    DisplayName text,
    LastAccessDate time,
    WebsiteUrl text,
    Location text,
    AboutMe text,
    Views number,
    UpVotes number,
    DownVotes number,
    ProfileImageUrl text,
    EmailHash text,
    AccountId number
)

CREATE TABLE PostFeedback (
    Id number,
    PostId number,
    IsAnonymous boolean,
    VoteTypeId number,
    CreationDate time
)

CREATE TABLE FlagTypes (
    Id number,
    Name text,
    Description text
)

CREATE TABLE Badges (
    Id number,
    UserId number,
    Name text,
    Date time,
    Class number,
    TagBased boolean
)

CREATE TABLE ReviewRejectionReasons (
    Id number,
    Name text,
    Description text,
    PostTypeId number
)

CREATE TABLE ReviewTaskResultTypes (
    Id number,
    Name text,
    Description text
)

CREATE TABLE Tags (
    Id number,
    TagName text,
    Count number,
    ExcerptPostId number,
    WikiPostId number
)

CREATE TABLE SuggestedEditVotes (
    Id number,
    SuggestedEditId number,
    UserId number,
    VoteTypeId number,
    CreationDate time,
    TargetUserId number,
    TargetRepChange number
)

CREATE TABLE ReviewTaskResults (
    Id number,
    ReviewTaskId number,
    ReviewTaskResultTypeId number,
    CreationDate time,
    RejectionReasonId number,
    Comment text
)

CREATE TABLE PendingFlags (
    Id number,
    FlagTypeId number,
    PostId number,
    CreationDate time,
    CloseReasonTypeId number,
    CloseAsOffTopicReasonTypeId number,
    DuplicateOfQuestionId number,
    BelongsOnBaseHostAddress text
)

Question: Number of users on SO.

SQL:
SELECT COUNT(*) FROM Users