There are no additional restrictions on how many Store cards or upgrades a player can have equipped at once, beyond the accessory and hand limits.

• From page 18: "Each player has two accessory slots and two hands." Store cards have a Storage Location (Accessory, One-Handed, or Two-Handed), and you are limited by these slots.
• There is no mention of any further restrictions on the number of Store cards or upgrades you can have equipped, other than these slot limits.

Summary:
Players are limited only by the number of accessory slots (2) and hands (2) for equipping Store cards. No other restrictions are specified in the provided rulebook context.

Source: page 18

Source: Core Rulebook (pp. 13, 18, 28, 29, 4)