Six games in total.
Armature's 'Dead Star' had already been announced as one of April's PS4 PlayStation Plus Instant Game Collection Games, (see impressions here and here), and now we now the five others. Also, on the PS4, there will be the one-time Wii U exclusive 'Zombi' (review here). Another obvious highlight is 'I Am Alive' on the PS3.
Other April PS Plus games include 'A Virus Named Tom' for the Vita, 'Savage Moon' for the PS3, and 'Shutshimi' for the Vita.
'Dead Star' is extremely promising, and since so many will have the game, that should help to create a large player base. I think both 'Zombi' and 'I Am Alive' are good games to get (for free or with a subscription service). Even so, it does seem like Sony is having a hard time coming up with six games to give each month.
Source: PlayStation Blog