The support stance recently changed (January). So some documentations have some update latency I'm guessing :)
The official and latest status can be found here: https://technet.microsoft.com/en-us/library/dn554248.aspx
And it states the following:
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1 - 100 RP Trusts
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More than 100 RP Trusts
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1 - 30 AD FS Nodes
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WID Supported
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Not supported using WID - SQL Required
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More than 30 AD FS Nodes
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Not supported using WID - SQL Required
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Not supported using WID - SQL Required
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So up to 100 trusts, you can use up to 30 ADFS with WID. If you want to use more than 30 ADFS servers, regardless of the number of trusts, you have to go with SQL. If you want more than 100 trusts, regardless of the number of server, it will be SQL.
Of course, if you want to use Token Replay Detection (if you have Claim Providers Trusts, the Token Replay Detection works only for Claim Providers different from the local AD), or if you want to use the SAML Artifact Resolution, you have to go with SQL.
Note that you can also start with WID and eventually move to SQL later if your specs change or if you reach one of the thresholds for WID. The other way around, SQL to WID is, as of today, still unsupported.
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