I want to host my tor hidden service on my Raspberry Pi. But is it safe? Will my ISP know that my network is always having access with tor? And can my IP address get exposed ? Should I use a VPN on my Raspberry Pi?
But is it safe?
like with every server: this depends on your setup/configuration.
Will my ISP know that my network is always having access with tor?
whether it is Tor or not, your ISP will always know to what servers you are connecting. - but it hardly makes a difference (from that point of view) if you are using Tor or running a hidden service... and definitively no difference if you are running a relay or hidden service.
perhaps have a look at (and play around with) https://www.eff.org/pages/tor-and-https.
And can my IP address get exposed?
by what? - if Tor suffers a security issue and you don't maintain/upgrade your server, probably yes... but see section 1, it depends on your setup, configuration, know-how, etc.
Should I use a VPN on my Raspberry Pi?
why? what do you want to achieve with a VPN?
--> i don't get it why again and again users want to combine Tor with a VPN?!? - in the best case this is just an additional possible point of failure; in the worst case it counteractes the Tor-usage and harms resp. destroys your privacy/anonymity.
once and for all: a VPN has nothing to do with anonymity! - it just encrypts your traffic from your device to the VPN-server (which is just another man-in-the-middle); and from there it is redirected "unencrypted" (resp. as much/little encrypted as it were without a VPN) to the destination. - therefore it may only be an improvement, if you strongly distrust your ISP, hotspot, etc. (but then you still have to trust the VPN-provider!).
and if you are using Tor anyways, bridges are the far better solution.
not really an answer to your question, but some important things regarding Tor and VPN: Is Tor like a VPN?