can i open more than one trading account with a broker?

It depends on the broker, most of them will allow you open several trading accounts but a few of them have a one account per client rule.

When opening several trading accounts remember that you need to use them to trade every once in a while or the broker may charge you an account inactivity fee.

Yes, you can and it makes sense to do so especially if your broker does not offer a central online wallet.

When you open more than one trading account, you can save funds you dont want to trade with in one, and only leave funds you intend to trade with in the other.

Leaving all your trading funds in one account can be risky, because if you mistakenly open a bad trade, your account balance can be wiped off by the time you realize the mistake.

Please may i know some of the brokers that offer this cental online wallet for holding funds not yet assigned to opening trades?

Certain brokers like FxPro, Tickmill, & HF Markets i cant remember all of them right now, when I do i will leave it in the comment section.

Not all brokers allow you to open more than one trading account but a majority of them do.

Yes brokers allow you open more than one account. For example you can have an account for the MT4 platform and another account for MT5