It’s up to the DM, but the system is written assuming a single god, with one exception.
After reading the character creation section and the piety section, it seems abundantly clear that the rules are assuming you only follow a single god, but it is never explicitly stated. I can offer a few examples, though there are more.
From the Character Creation section.
Many characters’ ideals come directly from their service to a god. Each god’s description in chapter 2 includes a table you can use to determine your hero’s ideal if your character serves that god, instead of (or in addition to) an ideal derived from your background.
On supernatural gifts:
A character in Theros begins with one supernatural gift chosen from those in this section. Work with the DM to decide where your character’s gift came from. Is it tied to the god you serve?
From the God of Theros section.
On the difference between heroes and mere mortals:
It’s far more common for a hero to be devoted to an individual god than it is for an ordinary mortal.
On starting out:
When you choose a god to worship as a beginning character, your piety score related to that god is 1. Your piety score increases by 1 when you do something to advance the god’s interests or behave in accordance with the god’s ideals.
On piety score:
As a general rule, you can expect to increase your piety by 1 during most sessions of play, assuming that you are following your god’s tenets.
Notably, piety score is always referred to singularly.
The Oracle may follow multiple gods.
From the description of the Oracle supernatural gift:
The gods seek mortal oracles to act as their agents. As a result, most oracles devote themselves to the service of a single god and learn to ignore the voices of all others. Occasionally, two gods agree to share the services of an oracle. Oracles who try to remain independent often find themselves pursued by the agents of evil gods who would bind them to the god’s service, if necessary.
With this gift, the rules explicitly mention following two gods, but this gift precludes the use of the traditional piety system:
If you choose the Oracle supernatural gift, you gain different rewards for your piety score, instead of the ones normally granted by your god.
The only section that explicitly mentions serving more than a single god has an entirely unique piety system associated with it.
The message here is clear: the intent is that you only benefit from a single set of piety derived blessings at a time.
But to be clear, a DM may allow a player to play a character who follows two gods and has to juggle their responsibilities to each. The issue with this is that the divine blessings given by the piety system are quite good. A wise DM would consider the balance implications of permitting a character to double-dip, so to speak, into these blessings. Notably, the +50 piety blessings for every god is a +2 to an ability score and its max – equivalent to an epic boon. Allowing one player to double dip here is so significant a benefit, mechanically minded players will almost certainly find a way to shoehorn it into their back story.