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