How many cards can I hold? (How do conflicting effects work?)

How many cards can I hold? Normally, I'd have 7, but card text trumps rules.

On my field, I have Null Profusion and Kruphix, God of Horizons:

• Thanks for the edit, Murgatroid. The cards are Null Profusion and Kruphix, God of Horizons
– Tony
Aug 16, 2016 at 19:46
• This is not exactly what the question ask but just in case you where wondering the maximum hand size only forces you to discard at the end of your turn. Don't think you have to discard your cards the moment you have three cards in hand with that black enchantment. Aug 17, 2016 at 13:49

It depends, whichever one entered the battlefield last is the one that takes effect.

When a permanent enters the battlefield it receives a timestamp. Then when determining the result of continuous effects (the kind generated by both Kruphix and Null Profusion) you apply them in timestamp order.

So if the Profusion entered last you will have a max hand size of 2, and if Kruphix entered last you won't have a maximum hand size. If they were to enter the battlefield at the same time then the Active Player (the player whose turn it currently is) decides what the order of the timestamps is.

613.6. Within a layer or sublayer, determining which order effects are applied in is usually done using a timestamp system. An effect with an earlier timestamp is applied before an effect with a later timestamp.

613.6c An object receives a timestamp at the time it enters a zone.

613.6j If two or more objects would receive a timestamp simultaneously, such as by entering a zone simultaneously or becoming attached simultaneously, the active player determines their relative timestamp order at that time.

613.10. Some continuous effects affect game rules rather than objects. For example, effects may modify a player’s maximum hand size, or say that a creature must attack this turn if able. These effects are applied after all other continuous effects have been applied. Continuous effects that affect the costs of spells or abilities are applied according to the order specified in rule 601.2f. All other such effects are applied in timestamp order. See also the rules for timestamp order and dependency (rules 613.6 and 613.7).

• @diego Awesome. Thanks! Another question: what if two things entered at the same time? E.g. If Kruphix and Null Profusion were now both 3CMC creatures, and I played Collected Company (which bring in two 3-or-lower-CMC creatures)
– Tony
Aug 16, 2016 at 19:52
• You may also want to quote 613.10, to show that timestamps apply to these effects even though they aren't part of the normal layer system. Aug 16, 2016 at 19:52
• @Tony "613.6j If two or more objects would receive a timestamp simultaneously, such as by entering a zone simultaneously or becoming attached simultaneously, the active player determines their relative timestamp order at that time." Aug 16, 2016 at 19:53
• I want to note that "active player" means "the player who is taking the turn", not "the player who controls the objects", which may lead to some unwanted outcomes if you cast Collected Company on your opponent's turn. Aug 16, 2016 at 19:56
• @diego Also, one more note: 613.6a is also relevant here. I'm not sure if referencing it is necessary, though. Aug 16, 2016 at 20:13

It depends on what order they entered the battlefield. Rule 613.10 says that we apply game rule-changing effects in timestamp order. Rule 613.6 explains how timestamps work. Whichever entered the battlefield later[1] has the later timestamp, which applies second and overrides the earlier effect.

613.10. Some continuous effects affect game rules rather than objects. For example, effects may modify a player’s maximum hand size, or say that a creature must attack this turn if able. These effects are applied after all other continuous effects have been applied. Continuous effects that affect the costs of spells or abilities are applied according to the order specified in rule 601.2f. All other such effects are applied in timestamp order. See also the rules for timestamp order and dependency (rules 613.6 and 613.7).

613.6. Within a layer or sublayer, determining which order effects are applied in is usually done using a timestamp system. An effect with an earlier timestamp is applied before an effect with a later timestamp.

[1] Technically, there are ways to "refresh" the timestamp of a permanent, so the order they entered the battlefield is not always the same as their timestamp order, but none of those ways applies in the situation as described.